[SCO-0003] Allow missing files in file providers (#75)

### Motivation

Fixes #66.

Read the proposal
[here](https://github.com/czechboy0/swift-configuration/blob/hd-sco-0003/Sources/Configuration/Documentation.docc/Proposals/SCO-0003.md).

### Modifications

Adds the proposal.

### Result

N/A

### Test Plan

N/A

Author: czechboy0

Sources/Configuration/Documentation.docc/Proposals/Proposals.md

@@ -38,3 +38,4 @@ If you have any questions, ask in an issue on GitHub.
 - <doc:SCO-NNNN>
 - <doc:SCO-0001>
 - <doc:SCO-0002>
+- <doc:SCO-0003>

Sources/Configuration/Documentation.docc/Proposals/SCO-0003.md

@@ -0,0 +1,136 @@
+# SCO-0003: Allow missing files in file providers
+
+Add an `allowMissing` parameter to file-based providers to handle missing configuration files gracefully.
+
+## Overview
+
+- Proposal: SCO-0003
+- Author(s): [Honza Dvorsky](https://github.com/czechboy0)
+- Status: **Implemented (1.0.0-alpha.1)**
+- Issue: [apple/swift-configuration#66](https://github.com/apple/swift-configuration/issues/66)
+- Implementation:
+    - [apple/swift-configuration#73](https://github.com/apple/swift-configuration/pull/73)
+- Revisions:
+    - v1 - Nov 12, 2025 - Initial proposal.
+
+### Introduction
+
+Add an `allowMissing` Boolean parameter to file-based configuration providers to enable graceful handling of missing configuration files.
+
+### Motivation
+
+Applications often need to handle optional configuration files that may not exist at startup or during runtime. Currently, all file-based providers (`FileProvider`, `ReloadingFileProvider`, `DirectoryFilesProvider`, and `EnvironmentVariablesProvider` when initialized from an `.env` file) throw errors when the specified configuration file is missing, which creates several challenges:
+
+- Applications fail to start when optional configuration files are missing, even when they could operate with sensible defaults specified in code.
+- In containerized environments or cloud deployments, configuration files may be mounted dynamically or created by other services, making their availability timing unpredictable.
+- Developers must create placeholder configuration files even when working on features that don't require external configuration.
+
+Currently, adopters must implement workarounds such as manually checking for a file's presence before creating a file-based provider, which requires writing needless boilerplate code.
+
+### Proposed solution
+
+We propose adding an `allowMissing` parameter to the initializers of `FileProvider`, `ReloadingFileProvider`, `DirectoryFilesProvider`, and `EnvironmentVariablesProvider`. When set to `true`, missing files are treated as empty configuration sources instead of causing initialization failures.
+
+Key behavioral changes:
+
+```swift
+// Current behavior - throws if config.json doesn't exist
+let provider = try await FileProvider<JSONSnapshot>(filePath: "config.json")
+
+// New behavior - succeeds even if config.json doesn't exist
+let provider = try await FileProvider<JSONSnapshot>(
+    filePath: "config.json",
+    allowMissing: true
+)
+```
+
+The `allowMissing` parameter defaults to `false`, preserving existing behavior for backward compatibility and keeping the strict variant as the default. When `true`:
+
+- Missing files are treated as empty configuration (no key-value pairs).
+- Reloading providers continue to work - providers detect when missing files are created, updated, and deleted.
+- Malformed files still throw parsing errors regardless of the `allowMissing` setting.
+- Directory provider treats missing directories as empty.
+
+Example usage patterns:
+
+```swift
+// Multi-layered configuration with optional overrides
+let config = ConfigReader(provider: [
+    EnvironmentVariablesProvider(),
+    try await FileProvider<JSONSnapshot>(
+        filePath: "optional-config.json",
+        allowMissing: true // Won't fail if missing
+    ),
+    InMemoryProvider(data: ["fallback": "values"])
+])
+
+// Reloading provider that handles dynamic file creation, updates, and deletion
+let dynamicConfig = try await ReloadingFileProvider<YAMLSnapshot>(
+    filePath: "/etc/dynamic/config.yaml",
+    allowMissing: true,
+    pollInterval: .seconds(5)
+)
+```
+
+### Detailed design
+
+#### API additions
+
+All affected initializers will gain an `allowMissing` parameter:
+
+```swift
+// FileProvider.swift
+public init(
+    snapshotType: Snapshot.Type = Snapshot.self, 
+    parsingOptions: Snapshot.ParsingOptions = .default, 
+    filePath: FilePath,
+    allowMissing: Bool = false // <<< new
+) async throws
+
+// ReloadingFileProvider
+public convenience init(
+    snapshotType: Snapshot.Type = Snapshot.self,
+    parsingOptions: Snapshot.ParsingOptions = .default,
+    filePath: FilePath,
+    allowMissing: Bool = false, // <<< new
+    pollInterval: Duration = .seconds(15),
+    logger: Logger = Logger(label: "ReloadingFileProvider"),
+    metrics: any MetricsFactory = MetricsSystem.factory
+) async throws
+
+// DirectoryFilesProvider
+public init(
+    directoryPath: FilePath,
+    allowMissing: Bool = false, // <<< new
+    secretsSpecifier: SecretsSpecifier<String, Data> = .all,
+    arraySeparator: Character = ",",
+    keyEncoder: some ConfigKeyEncoder = .directoryFiles
+) async throws
+
+// EnvironmentVariablesProvider
+public init(
+    environmentFilePath: FilePath,
+    allowMissing: Bool = false, // <<< new
+    secretsSpecifier: SecretsSpecifier<String, String> = .none,
+    bytesDecoder: some ConfigBytesFromStringDecoder = .base64,
+    arraySeparator: Character = ","
+) async throws
+```
+
+#### Configuration keys
+
+When using `ConfigReader`-based initialization, a new key is supported:
+
+- `allowMissing` (boolean, optional, default: false): Whether to allow missing files/directories.
+
+### API stability
+
+This change is purely additive, so no existing adopters are affected.
+
+### Future directions
+
+Nothing comes to mind at the moment.
+
+### Alternatives considered
+
+Status quo - we could have kept the file-reading behavior strict, which would require adopters to write conditional logic when setting up their `ConfigReader`.