Response headers in didFinishUpload delegate + data race fix (supersedes #167)

[LefterisHaritou]

Summary

Adds HTTP response header support to the didFinishUpload delegate callback, and fixes a data race in TUSAPI.

This continues and fixes the work started in #167 (by @srvarma7), rebased on top of the latest main (including the progress-wiring changes from #228).

What's included

• Expose response headers on completion. A new didFinishUpload(id:url:context:client:responseHeaders:) delegate method surfaces the server's response headers. The previous signature is kept as a deprecated compatibility shim that forwards to the new method, so existing conformers keep working without changes.
• New HTTPURLResponse+Headers extension to extract headers into a [String: String] dictionary.
• Propagate headers into metadata. UploadDataTask now unpacks the upload result as (offset, responseHeaders) and stores responseHeaders on UploadMetadata.
• Tolerant decoding / fix data racing. UploadMetadata decodes responseHeaders with decodeIfPresent, so older persisted metadata without that key won't fail to decode. TUSAPI callback handling was adjusted to avoid a data race on the shared callback state.

Relationship to #167

This supersedes #167 with the missing fix applied and a clean rebase onto current main. Happy to close #167 in favor of this if preferred.

Testing

• swift build succeeds (only the expected deprecation warning from the intentional compatibility shim).