Add TokenSourceRequest / TokenSourceResponse messages

[1egoman]

Per this pull request comment, add the new TokenSource request and response types into this repository so they can be encoded / decoded as snake case more elegantly in the js + swift clients, soon hopefully more.

This is the first time I've committed to this repository - I'm not sure if I put these in the right place / etc. Let me know if they should go in some other file!

[changeset-bot]

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[1egoman]

Slack comment from @lukasIO:

I would vote to move the TokenSource out of the room.proto file and either into models or into a new livekit_token_source file.
the room file is mostly concerned with signal request types that are sent between client and server.

Done - moved these messages to a new protobufs/livekit_token_source.proto file.

[1egoman]

Slack comment from @lukasIO:

jfyi for the JS package build we’ll have to manually add the file also here https://github.com/livekit/protocol/tree/main/packages/javascript/src in both the index.js and d.ts file

This should be done.

[lukasIO]

lgtm!