Building fully balanced transactions for users

Hi all,

I'm having a use case where a Service builds a Cardano transaction, given some Client provided inputs. The idea is that the Client only needs access to a Wallet to add a signature and finally submit the transaction that the Service built.

I wonder how can I achieve this with BPI...

1. Client provides their Wallet's PublicKeyHash, Collateral and Ada change TxOuts to the Service,
2. The Service builds a fully balanced transaction and adds their signature as well,
3. The Service returns the half-signed transaction (just CBOR) to the Client,
4. The Client takes the transaction and uses their Wallet to add an additional signature and submit.

It's important to note that we want to build the transaction fully on the Service side, and that way keep our Clients very lightweight (ie. only requiring a Wallet).