[backport#13932] Implement joinpsbts RPC and tests

Summary:
Adds a joinpsbts RPC which combines multiple distinct PSBTs into
one PSBT.

https://github.com/bitcoin/bitcoin/pull/13932/commits/08f749c9147a5f3fdbbd880e0974b97084429002

Depends on D6043

This is a partial backport of Core PR13932 plus a test case

Test Plan:

ninja check
test_runner.py rpc_psbt

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D6044