Refactor transaction creation and transaction funding logic

Summary:
In preparation for more create transaction and fund transcation RPCs,
refactor the transaction creation and funding logic into separate
functions.

This is a partial backport of Core PR13557 : https://github.com/bitcoin/bitcoin/pull/13557/commits/58a8e28918025c28f19ba19cbaa4a72374162942

Test Plan:

make check
./test/functional/test_runner.py

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D4288