[backport#16185 1/3] gettransaction: add an argument to decode the transaction

Summary:
This adds a new boolean parameter 'decode' to the gettransaction call, which, if set to true, add a 'decoded' field to the result containing the decoded transaction

https://github.com/bitcoin/bitcoin/pull/16185/commits/7f3bb247a811582d1aa4805d8e601c19808dc7ba

Partial backport of Core PR16185

Test Plan:

ninja check check-functional

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D7370