[backport#15638] [build] Add several util units

Summary:
Adds the following util units and adds them to libbitcoin_util:

• util/url.cpp takes urlDecode from httpserver.cpp
• util/error.cpp takes TransactionErrorString from node/transaction.cpp and AmountHighWarn and AmountErrMsg from ui_interface.cpp

- util/fees.cpp takes StringForFeeReason and FeeModeFromString from policy/fees.cpp
- util/rbf.cpp takes SignalsOptInRBF from policy/rbf.cpp

• 'util/validation.cpp` takes FormatStateMessage and strMessageMagic from 'validation.cpp`

https://github.com/bitcoin/bitcoin/pull/15638/commits/91a25d1e711bfc0617027eee18b9777ff368d6b9

Depends on D5994

This is a partial backport of Core PR15638

Test Plan:

ninja check-all

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D5997