Fix message signature verification

[pcapriotti]

This PR reorganises the MLS Message type to support correct signature verification. The new Message type is now composed of a TBS and a separate signature (together with unsigned extra fields). In the case of a ciphertext message, the signature and extra fields are empty.

This supersedes parts of #2567, but it does not contain any code for supporting external proposals in Galley.

Tracked by https://wearezeta.atlassian.net/browse/FS-435.

Checklist

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