Import protocol crate from session-sharing-server repo.

[vorporeal]

This yanks the session-sharing-protocol crate from https://github.com/warpdotdev/session-sharing-server into this dedicated repository.

The client compiles against this protocol, so when we open-source the client, we'll need to make the protocol code public too. We don't want to make the server code public, though, so they need to be in separate repos (hence this PR).

Once this merges, I'll send out a pair of follow-up PRs to update the client repo to point at the crate here, and the server repo to remove the crate from there and instead point at it here.

[vorporeal]

• Import protocol crate from session-sharing-server repo. #1 👈 (View in Graphite)
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