Refactor rawtransaction's SignTransaction into generic SignTransaction function

Summary: Backport of Core PR18115 part [1/9] : https://github.com/bitcoin/bitcoin/pull/18115/commits/2c52b59d0a44a86d94fee4e437978d822862c542

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, PiRK

Reviewed By: PiRK

Differential Revision: https://reviews.bitcoinabc.org/D8094