Add sending to returned values from async Storage APIs

[morganchen12]

This allows returned values to be used in async contexts. Otherwise, the following example:

let root = Storage.storage().reference()

// Get metadata properties
do {
  let metadata = try await root.getMetadata()
} catch {
  // ...
}

Produces an error:

Non-sendable type 'StorageMetadata' returned by implicitly asynchronous call to nonisolated function cannot cross actor boundary

Annotating these values as sending requires no other conformance work since references to the returned values are not held by Storage. See the sending proposal for more details.

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Apple API Diff Report

Commit: 909266e
Last updated: Wed Jan 8 16:05 PST 2025
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[BUILD ERROR] FirebaseStorage

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[morganchen12]

Closed in favor of #14763