minor feerate improvements

Summary:
This is a partial backport of core#20220

Most of the PR is not applicable because we do not use estimate_mode nor conf_target for fees.

List of changes and source commits:

• Use a const ref in FundTransaction for options: https://github.com/bitcoin/bitcoin/pull/20220/commits/1697a40b6f841a54ee0d9744ed7fd09034b0ddad
• Minor rpc_psbt.py refactoring: https://github.com/bitcoin/bitcoin/pull/20220/commits/6e1ea4273e52fdcd86c87628aa595c03a071ca8c
• Add a log.info in wallet_basic.py: https://github.com/bitcoin/bitcoin/pull/20220/commits/dd341e602d5160fc621c0299179b91403756b61d
• Improve RPC doc for subtract_fee_from_outputs: https://github.com/bitcoin/bitcoin/pull/20220/commits/778b9be40667876c705e586849ea9c9e44cf451c

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D10716