[1.x] Externalize group state storage, an alternative

[mulmarta]

Problem:

We want to remove the GroupStateStorage trait in favor of application managing storage. This is a possible alternative to #211.

Solution:

A group member will use the following code to process a proposal_message and a commit_message that includes proposal_message by reference.

// Receive proposal
let ProtocolMessage {
    group_id: &'a [u8],
    epoch_id: u64,
    content_type: ContentType::Proposal,
} = proposal_message.description()
else {
    // Do something else
};

let epoch = my_storage.load_epoch(group_id, epoch_id)?;

// Decrypt and verify auth of proposal
let proposal: Proposal = client.process_proposal(&epoch, proposal_message)?;

// Receive commit
let ProtocolMessage {
    content_type: ContentType::Commit,
    ..
} = commit_message.description()
else {
    // Do something else
};

// Currently `new_epoch_builder` would be `commit_processor`
let commit_effect = client
    .new_epoch_builder(epoch, commit_message)?
    .with_proposal(proposal)
    .with_psk(my_storage.load_psk(builder.required_psks()[0])?)
    .override_proposals(my_proposal_rules(builder.proposals())?)
    .build()?;

// Delete epoch for that group. Store updated epoch and past epoch for late application messages
my_storage.store_epoch(commit_effect.new_epoch, commit_effect.past_epoch)?;

where

CommitEffect {
    epoch: Option<GroupState>, // None if we're removed
    past_epoch: PastEpoch,
    // other info
}