feat(prebuilt): allow custom footer in participant side pane

[aldenquimby]

Description

Fixes #3446

Allows prebuilt consumers to render a custom react component at the bottom of the participant list side pane:

Implementation note, gotchas, related work and Future TODOs (optional)

• I could not figure out where types-prebuilt is coming from
• I suspect using ReactNode as the type may be problematic b/c that library doesn't have react as a dependency right now

Pre-launch Checklist

• The Documentation is updated accordingly, or this PR doesn't require it.
• I updated/added relevant documentation.
• I listed at least one issue that this PR fixes in the description above.
• I added new tests to check the change I am making, or this PR is test-exempt.
• All existing and new tests are passing.

Merging:

• Squash merge to dev
• Merge commit to publish-alpha and main

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