#4082 add mutable accessors to QuorumProposalWrapper

[pls148]

Closes #4082

This PR:

• Adds mutation methods on QuorumProposalWrapper
• Addresses a missed comment from #4093 add epoch and friends to commitments of new x2 objects #4094

This PR does not:

Key places to review:

[jparr721]

Curious, why bother adding mutable and immutable accessors when we could just make all the fields pub? Seems like this is just adding indirection.

[bfish713]

Yeah I feel like all these fields do not need mutable access

[ss-es]

I think it's not clear we need all of these, but no objection from me

I think QuorumProposalWrapper is really a HotShot-internal type, and it makes sense to me to leave the fields private (or pub(crate) at most)