[FS-509] Bare Proposal Support

[mdimjasevic]

This patch implements the support for bare proposals in MLS.

Tracked by https://wearezeta.atlassian.net/browse/FS-509.

Things that are yet to be finished:

• In Galley.API.MLS.Message in function applyProposalRef check that the add proposal's group ID and epoch number are in line with those of the conversation. Actually, it is not clear if they should be checked against the epoch number when the proposal is received or when it is committed,
• In Galley.API.MLS.Message make use of the checkProposal function when processing inlined proposals,
• See if better errors (e.g., potentially new and/or statically known errors) can be used in proposal-proccessing code introduced by this PR,
• Add two Galley integration tests in API.MLS for commits. In the commit group there should be two more tests, one successfully referencing an existing proposal, and one failing test referencing a non-existing proposal,
• Finish Galley integration tests in API.MLS for proposals. So far there's only one finished test that tries to send an add proposal to a conversation that doesn't exist in Galley. Other tests that are yet to be implemented vacuously succeed (as denoted by pure () in the proposals test group).

Checklist

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