Implement StarboardRenderer.

This is an implementation of ::media::Renderer via Starboard. It is not
hooked up to production yet, but it will eventually replace CastRenderer
for starboard-based linux cast builds.

Test: Added unit tests

Bug: b:323610278
Change-Id: Ic394dc8b5a56472185fc2471a612de5e1727f231
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6616773
Commit-Queue: Antonio Rivera <antoniori@google.com>
Reviewed-by: Shawn Quereshi <shawnq@google.com>
Cr-Commit-Position: refs/heads/main@{#1475158}