Add privacy manifest (#2695)

Motivation:

NIOPosix and NIOFileSystem use stat(2) (or some variation of it). These
are "required reason APIs" meaning that their usage and reason must be
declared in a privacy manifest in order to be submitted to the App Store.

Modifications:

- Add a privacy manifest to NIOPosix and NIOFileSystem

Result:

Apps using NIO won't be rejected from App Store submission for missing
privacy manifests.

.PrivacyInfo.xcprivacy

@@ -0,0 +1,23 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
+<plist version="1.0">
+<dict>
+	<key>NSPrivacyTracking</key>
+	<false/>
+	<key>NSPrivacyAccessedAPITypes</key>
+	<array>
+		<dict>
+			<key>NSPrivacyAccessedAPITypeReasons</key>
+			<array>
+				<string>0A2A.1</string>
+			</array>
+			<key>NSPrivacyAccessedAPIType</key>
+			<string>NSPrivacyAccessedAPICategoryFileTimestamp</string>
+		</dict>
+	</array>
+	<key>NSPrivacyCollectedDataTypes</key>
+	<array/>
+	<key>NSPrivacyTrackingDomains</key>
+	<array/>
+</dict>
+</plist>

Package.swift

@@ -23,6 +23,14 @@ let swiftSystem: PackageDescription.Target.Dependency = .product(
   condition: .when(platforms: [.macOS, .iOS, .tvOS, .watchOS, .linux, .android])
 )
 
+// This doesn't work when cross-compiling: the privacy manifest will be included in the Bundle and
+// Foundation will be linked. This is, however, strictly better than unconditionally adding the
+// resource.
+#if canImport(Darwin)
+let includePrivacyManifest = true
+#else
+let includePrivacyManifest = false
+#endif
 
 let package = Package(
     name: "swift-nio",
@@ -83,7 +91,8 @@ let package = Package(
                 "NIOCore",
                 "_NIODataStructures",
                 swiftAtomics,
-            ]
+            ],
+            resources: includePrivacyManifest ? [.copy("PrivacyInfo.xcprivacy")] : []
         ),
         .target(
             name: "NIO",
@@ -199,6 +208,7 @@ let package = Package(
                 swiftSystem,
             ],
             path: "Sources/NIOFileSystem",
+            resources: includePrivacyManifest ? [.copy("PrivacyInfo.xcprivacy")] : [],
             swiftSettings: [
                 .define("ENABLE_MOCKING", .when(configuration: .debug))
             ]

Sources/NIOFileSystem/PrivacyInfo.xcprivacy

@@ -0,0 +1 @@
+../../.PrivacyInfo.xcprivacy

Sources/NIOPosix/PrivacyInfo.xcprivacy

@@ -0,0 +1 @@
+../../.PrivacyInfo.xcprivacy