Create utility RPCs for PSBT

Summary:
decodepsbt takes a PSBT and decodes it to JSON

combinepsbt takes multiple PSBTs for the same tx and combines them.

finalizepsbt takes a PSBT and finalizes the inputs. If all inputs
are final, it extracts the network serialized transaction and returns
that instead of a PSBT unless instructed otherwise.

createpsbt is like createrawtransaction but for PSBTs instead of
raw transactions.

convertpsbt takes a network serialized transaction and converts it
into a psbt. The resulting psbt will lose all signature data and
an explicit flag must be set to allow transactions with signature
data to be converted.

This is a partial backport of Core PR13557 : https://github.com/bitcoin/bitcoin/pull/13557/commits/c27fe419efb3b6588c400d764122ffb33375e028

Test Plan: Unfortunately, the test have all been dumped at once at the end.

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D4351