Return one batch proof per storage map on `GetAccount`

[igamigo]

On the GetAccountProofs endpoint, we are returning a list of the requested storage map keys with an SMT proof that can be used on the client side to provide data to the VM in order to execute transactions with foreign account data. This is fine but we could optimize bandwidth and processing on the client side by returning one batch proof per map instead.

More context: #598 (comment)

[drahnr]

What's our definition of batch proof in this scope? We are currently employing merkle paths as proofs for individual storage items, which makes this work, but what's our plan for compounding them into a single proof.

[bobbinth]

What's our definition of batch proof in this scope? We are currently employing Merkle paths as proofs for individual storage items, which makes this work, but what's our plan for compounding them into a single proof.

As discussed offline, in this context, we want to avoid sending duplicate Merkle path nodes (e.g., nodes close to the top of the tree are likely to be shared across multiple paths). To accomplish this, we probably want to have a message that can describe a PartialSmt (@igamigo has recently implemented serialization for it in miden-crypto).