PSBT Support with Miniscript

[storopoli]

The policy can be something like:

tr(UNSPENDABLE, {
  multi_a(2, npub_1, npub_2), {
    and_v(
      v:multi_a(2, npub_1, npub_arb),
      older(timelock_duration)
    ),
    and_v(
      v:multi_a(2, npub_2, npub_arb),
      older(timelock_duration)
    )
  }
})

Needs to figure out how to sign the fucking PSBT with HashMap<PublicKey, PrivateKey> see rust-bitcoin/rust-bitcoin#4150

[jaonoctus]

Should be doable with this code now:

rust-bitcoin/rust-bitcoin#4150 (comment)

[luisschwab]

After #46 is merged we can do this.

[storopoli]

Won't be trivial, but definitely doable still.
We need to change all the unsigned_tx or signed_tx in the UI and backend for PSBTs

[storopoli]

First step is to see if you can change from Transaction to Psbt in this test:

scrow/src/sign.rs

Lines 221 to 222 in 5a85616

	#[test]
	fn sign_collaborative_tx_flow() {

Then, we know that it f*** works.