- Introduce a `FeeRateFormat` enum and change `CFeeRate::ToString()`
to use it for `BTC/kvB` vs `sat/vB` output formatting.
- Handle all enum values, hence remove default case in `CFeeRate::ToString()`
and `assert(False)` when a `FeeRateFormat` value is not handled.
- Keep `FeeEstimateMode` focused on fee estimation behavior by removing fee rate format
values from `FeeEstimateMode`.
- Update all formatting call sites and tests to pass `FeeRateFormat` explicitly, separating fee rate format
from fee-estimation mode selection.
Move enum and message formatting functions to a common/messages header where
they should be more discoverable, and also out of the util library, so they
will not be a dependency of the kernel
The are no changes in behavior and no changes to the moved code.
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`
