Implement get_foreign_account_inputs() for TransactionInputsDataStore

[sergerad]

After #1278 is merged.

As per comment:

   fn get_foreign_account_inputs(
      &self,
      foreign_account_id: AccountId,
      _ref_block: BlockNumber,
  ) -> impl FutureMaybeSend<Result<AccountInputs, DataStoreError>> {
      async move { Err(DataStoreError::AccountNotFound(foreign_account_id)) }
  }

Returning an error is fine for now, but we'll need to figure out how to read AccountInputs for foreign accounts from TransactionInputs (the data should be in the advice_inputs - but it may not be simple to extract).

And comment:

if self.tx_inputs.account().id() != account_id {
              return >Err(DataStoreError::AccountNotFound(account_id));
          }

Similar to the above comment, we should be able to serve the foreign account requests. Maybe the issue we need is about the ability to support foreign account invocation during re-execution.

[sergerad]

we'll need to figure out how to read AccountInputs for foreign accounts from TransactionInputs (the data should be in the advice_inputs - but it may not be simple to extract).

@bobbinth any advice on how this extraction might work? All the fields in AdviceInputs seem to boil down to Word/Felt.

pub struct AdviceInputs {
    pub stack: Vec<Felt>,
    pub map: AdviceMap,
    pub store: MerkleStore,
}

[bobbinth]

The code that adds foreign account inputs into the advice provider is located here - and we'll need to do the reverse of that (i.e., we'll need to read AccountInputs from the advice provider). This involves two things:

First, we need to read PartialAccount from the advice map. To get the account header we'll need to get the elements located under account_id_key in the map, and then parse them into AccountHeader. Once we have the header, we can read AccountStorageHeader from the map in a similar way, and also get AccountCode from TransactionInputs.foreign_account_code.

After we build a PartialAccount, we'll also need to get AccountWitness. The main thing there is the Merkle path, and this one will need to come from the Merkle store. To do this, we'll need to use MerkleStore::get_path() method passing in account tree root (from the reference block) as the root and deriving index from account_id (for this, we should probably add a simple conversion function in miden-base - it could build on account_id_to_smt_key() function).

Overall, this process is pretty involved, and I'm wondering how much of it should live in miden-base because since the logic of putting AccountInputs into the advice provider is there, it probably makes the most sense to put the logic for reading AccountInputs from the advice provider there too.