Adds a joinpsbts RPC which combines multiple distinct PSBTs into
one PSBT.
https://github.com/bitcoin/bitcoin/pull/13932/commits/08f749c9147a5f3fdbbd880e0974b97084429002
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Depends on D6043
This is a partial backport of Core [[https://github.com/bitcoin/bitcoin/pull/13932 | PR13932]] plus a test case