Add App Context extension

cloudevents/extensions/appcontext.md

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+# App Context
+
+<!-- no-verify-translation -->
+
+This extension gives information about a mobile app that triggered an event.
+This is a way to authenticate the app that triggered an event, orthogonally
+to a source.
+
+For example, in an end-user accessible backend like a Supabase or Firebae database,
+the source of a data change event is the database. But the authentication for that
+write might be the user (covered by the [authcontext](./authcontext.md) extension)
+and the app that triggered the event (covered by this extension).
+
+Common uses can be to track engagement with a particular app, or to
+allow a backend to enforce policies based on the app that triggered
+an event. For example, either the user or the app can be used to identify
+whether an AI API could have high or low rate limits, expensive or cost effective
+models, etc.
+
+## Notational Conventions
+
+As with the main [CloudEvents specification](../spec.md), the key words "MUST",
+"MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT",
+"RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as
+described in [RFC 2119](https://tools.ietf.org/html/rfc2119).
+
+However, the scope of these key words is limited to when this extension is
+used. For example, an attribute being marked as "REQUIRED" does not mean
+it needs to be in all CloudEvents, rather it needs to be included only when
+this extension is being used.
+
+## Attributes
+
+### appid
+- Type: `String`
+- Description: A cloud provider opaque string identifying the mapping of a given
+  app to a given backend.
+- Constraints
+  - OPTIONAL
+
+### appdisplayname
+- Type: `String`
+- Description: How the app appears to users.
+- Constraints
+  - OPTIONAL
+
+### androidpackagename
+- Type: `String`
+- Description: The Android package name of the app triggering the event.
+  It MUST follow Android package naming conventions: it MUST have at least two segments,
+  separated by periods (`.`); each segment MUST start with a letter; and all characters
+  MUST be alphanumeric or an underscore (`[a-zA-Z0-9_]`).
+- Constraints
+  - OPTIONAL
+  - MUST be present if the app is an Android app.
+
+### iosbundleid
+- Type: `String`
+- Description: The Apple bundle identifier of the app triggering the event.
+  It MUST follow Apple bundle naming conventions: it MUST contain only alphanumeric
+  characters (`A-Z`, `a-z`, `0-9`), hyphens (`-`), and periods (`.`), typically adopting
+  a reverse-DNS format.
+- Constraints
+  - OPTIONAL
+  - MUST be present if the app is an iOS app.