[MLS Spec change] Ensure that a signature public key is always intelligible #637

[franziskuskiefer]

Description of the changes
This PR moves the signature key from Credential to LeafNode, to ensure that an MLS client always has a signature public key with which to verify messages, even if it does not support a member's credential type.

This is an alternative approach to mlswg/mls-protocol#628, and IMO a bit cleaner. We no longer need the notion of a SignatureScheme, since BasicCredential doesn't stand alone. And since Credential only appears in LeafNode, common fields can be hoisted up into LeafNode. The Credential struct really only exists as a conceptual boundary, to make the exposition clearer; otherwise you could just shove the select into LeafNode.

Note that this does not obviate the need for mlswg/mls-protocol#631. This PR only ensures that the signatures can be processed; that PR helps ensure that the signatures are meaningful.

Link to the exact changes
mlswg/mls-protocol#637

[franziskuskiefer]

This has been addressed as part of #819 and #1222.
No changes are needed for this spec change but x509 will be implemented in #134 and address any changes introduced in this change.