add multi-asset support to new `sendall` RPC

I am in the process of merging https://github.com/bitcoin/bitcoin/pull/24118 into Elements.

This PR adds a new `sendall` RPC. However, I am just merging the bitcoin version, and I am not adding elements-specific arguments.

After this is merged, we will need to do follow-up work which adds an `asset` argument to `sendall`, so that a user can choose to `sendall` of a specific asset, rather than simply sending all the UTXOs in their wallet, regardless of asset-type.