Implement PSBT Structures and un/serialization methods per BIP 174

Summary: This is a partial backport of Core PR13557 : https://github.com/bitcoin/bitcoin/pull/13557/commits/41c607f09badb2c3ed58ff6fb17a8ebbef2cdabd

Test Plan: Unfortunately, The PR dump all the tests at once at the end, so there are n test for the new code :/

Reviewers: #bitcoin_abc, Fabien, jasonbcox

Reviewed By: #bitcoin_abc, Fabien, jasonbcox

Subscribers: jasonbcox, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D4258