Make dependencies optional in VideoFrameMetadata

which allows the W3C API to make them optional based on the presence
of the DD (or GFD) extension as described in
  https://w3c.github.io/webrtc-encoded-transform/#dom-rtcencodedvideoframemetadata-dependencies

This changes the signature from
  std::span<const int64_t> GetFrameDependencies() const;
which will soon be deprecated to
  std::optional<std::span<const int64_t>> GetDependencies() const;
The same applies to the setter.

Bug: webrtc:515776877
Change-Id: I113134361943bd28593afd56dae2fce9550c9951
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/475000
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Reviewed-by: Guido Urdaneta <guidou@webrtc.org>
Commit-Queue: Philipp Hancke <philipp.hancke@googlemail.com>
Cr-Commit-Position: refs/heads/main@{#47802}