More powerful tx construction API

[LLFourn]

gun send only allows single address sends but we should allow building arbitrary transactions.

I'm not sure what the CLI api should be but something like:

gun psbt build \
    -i 931043e6794c465665684fe0ef8c3ed070502bddd58ec7b07fe3aa2a9bbe3bf4:0 \
    -i 2a9cd9d7ff02f0863335cbbe1908ad2b263935ddbad555b1b806ab168c22d7a7:1 \
    -o bc1qw6h3w0g2446j2ycfe8h87sufethtawmsut8s9t:0.1BTC \
    -o bc1q07sq6ch5gajwcr05c7nsdmegnyfsuw3lawgwul:0.3BTC \
    --fee rate:5.1

This would output a PSBT.

You'd then have a gun psbt sign command to sing it and a gun psbt broadcast to check if it's finalized and try and broadcast it.

This is necessary to use gun as part of a traditional multisig setup where it isn't the initiating signer. i.e. where you just have a gun on a laptop and get PSBTs and sign them from there.