Consider adding efficient structure for client generate of multiple annotated commits

Currently the entire AnnotatedCommit structure needs to be generated separately for each Partial MLS client, even though only the `receiver_membership_proof` and possibly the `resolution_index` change from one Partial MLS client to another.

Consider creating a structure the clients can send which allows a committing client to generate all the proofs at once and then only send the part required to partial mls client.