Add PSBT keyspend signing routines

[instagibbs]

All that's required of libwally for CLN taproot address support.

[jgriffiths]

ack 5865f8c8bde622b449a49a8b4146336cbe9c0a2a. Will merge with rebase and the last commit split and squashed into the code and test commits.

[jgriffiths]

Merged, I added roundtrip tests for the taproot signature as well.