HTTPResumableUpload contains the core logic. It uses an event loop to
synchronize its state internally. Some methods are safe to call from off
of that event loop and have been moved to a new sendable view. The
HTTPResumableUpload type is marked as explicitly not sendable.
As such, most other types now hold on to the sendable view and use that
as the interface to HTTPResumableUpload.
HTTPResumableUploadChannel must be sendable (it's a Channel) and now
uses safe abstractions (where possible).
Support HTTP resumable upload.
### Motivation:
Supporting HTTP resumable upload protocol defined in
https://datatracker.ietf.org/doc/html/draft-ietf-httpbis-resumable-upload-05
* Interop version 3: iOS 17.0, macOS 14.0
* Interop version 5: iOS 18.0, macOS 15.0
* Interop version 6: iOS 18.1, macOS 15.1
### Modifications:
2 new public classes, `HTTPResumableUploadHandler` and
`HTTPResumableUploadContext`, and a few other supporting objects to
manage resumable uploads and translate them into regular uploads.
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Co-authored-by: Jonathan Flat <jflat@apple.com>
Co-authored-by: Cory Benfield <lukasa@apple.com>
