Output descriptors module

Summary:
This is a partial backport of Core PR13697 : https://github.com/bitcoin/bitcoin/pull/13697/commits/fe8a7dcd78cfeedc9a7c705e91384f793822912b

Also includes:

• Descriptor tests: https://github.com/bitcoin/bitcoin/pull/13697/commits/0652c3284fe12941b28624dbbf5e0862c5d0dbc3

Test Plan:

make check

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D4420