Add asynchronous image decoding support

[thliu21]

Summary

• add AsyncImageDecoding for decoders that need to call asynchronous APIs
• await async decoders on the existing image decoding queue while preserving error wrapping, cancellation, priority, and signpost behavior
• keep existing synchronous ImageDecoding implementations source-compatible
• document async decoder registration and the synchronous cache API limitation
• add coverage for successful async decoding and error propagation

Motivation

ImageDecoding currently exposes only a synchronous entry point, so implementations backed by async APIs have to block a thread with a semaphore. This adds a dedicated async refinement that the pipeline can detect and await without changing existing decoders.

Closes #860.

Testing

• swift build (all package targets)
• ImagePipelineDecodingTests: 5 passed
• ImagePipelineTests and ImagePipelineProgressiveDecodingTests: 35 passed
• DocC build

The macOS test runs used Thread Sanitizer disabled because the local Xcode 26.3 test runner could not load its TSan runtime; the shared scheme remains unchanged.