Registers `MediaSource` objects to the `MediaSourceRegistry` through a
`MediaSourceAttachment` proxy. The new `MediaSourceAttachment` class is
cross-thread reference counting safe, unlike `MediaSource`, which will
allow for utilizing `MediaSource` objects created on Dedicated Workers.

This change does not add any additional functionality, and is intended
as an incremental update towards enabling the creation of `MediaSource`
objects on Dedicated Worker threads.

See [Chromium's `MediaSourceAttachment` interface][1], and the commit
that [initially introduced it][2].

[1]: https://source.chromium.org/chromium/chromium/src/+/main:third_party/blink/renderer/core/html/media/media_source_attachment.h
[2]: https://chromium-review.googlesource.com/c/chromium/src/+/2300804

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