Implement GetKey for KeyMap

[LLFourn]

See the trait here:

https://docs.rs/bitcoin/latest/bitcoin/psbt/trait.GetKey.html

I think it'd make sense if KeyMap (or something like it) implemented this trait so you could sign PSBTs with the secret keys extracted from a private descriptor.

[apoelstra]

Oo, neat. Concept ACK. May be a little bit because we are rejigging the relationship between rust-bitcoin and rust-miniscript and I'm unsure what happens to PSBT during that transition.

[tcharding]

Once bitcoin depends on miniscript hopefully we move psbt_v2 into bitcoin but that shouldn't effect this issue, we can solve this in either miniscript::psbt and port it or in psbt_v2 if we have done the move already. (psbt_v2 contains all the code from psbt::miniscript anyways.)