Client/Server Multisig Setup with Beet

**Is your feature request related to a problem? Please describe.**
I want to be able to setup multisig system that consists of a client (beet) and a server. Both need to sign a transaction.

**Describe the solution you'd like**
The process I have in mind looks like this:
1. The server creates a raw (unsigned transaction)
2. Sever sends it to the client
3. Clients signs the raw transaction (one out of two require signatures) using beet
4. Client sends the partially signed tx to server
5. Server adds the second signature and broadcasts the transaction

**Describe alternatives you've considered**
The alternative would be to do it the other way round and let the server sign first. The difference should be minimal as you just have to keep track of a potentially existing signature in the to-be-signed transaction.

**Additional context**
We would like to be able to improve security by removing the potential of the server-installed private key to do anything without the approval of a local signature.