UI: Add support for additional canvases to multitrack video output

[palana]

This PR depends on #10633

Description

Adds frontend API calls to allow plugins to register additional canvases/video outputs that will participate in multitrack video configuration generation and multitrack video streams

Motivation and Context

Currently additional views/canvases have to use separate rtmp connections or similar to be streamed, where the multitrack video output allows both multiple renditions of the same view and also different views to be streamed

How Has This Been Tested?

Twitch Enhanced Broadcasting beta

Types of changes

• New feature (non-breaking change which adds functionality)

Checklist:

• My code has been run through clang-format.
• I have read the contributing document.
• My code is not on the master branch.
• The code has been tested.
• All commit messages are properly formatted and commits squashed where appropriate.
• I have included updates to all appropriate documentation.