Support manually selecting inputs consuming their entire value

This commit introduces an alternative way of splicing in funds without
coin selection by requiring the full UTXO to be provided. Each UTXO's
entire value (minus fees) is allocated towards the channel, which
provides unified balance wallets a more intuitive API when splicing
funds into the channel, as they don't particularly care about
maintaining a portion of their balance onchain.

To simplify the implementation, we require that contributions are not
allowed to mix coin-selected inputs with manually-selected ones. Users
will need to start a fresh contribution if they want to change the
funding input mode.

Author: wpaulino