Merge bitcoin/bitcoin#34552: fees: refactor: separate feerate format from fee estimate mode

c1355493e2 refactor: fees: split fee rate format from fee estimate mode (ismaelsadeeq)
922ebf96ed refactor: move-only: move `FeeEstimateMode` enum to `util/fees.h` (ismaelsadeeq)

Pull request description:

  ### Motivation

  Part of #34075

  - The `FeeEstimateMode` enum was responsible for both selecting the fee estimation algorithm and specifying the fee rate' format.

  ####  Changes in this PR:
     * The `FeeEstimateMode` enum (`UNSET`, `ECONOMICAL`, `CONSERVATIVE`) is moved to a new util/fees.h header.
     * A new `FeeRateFormat `enum (`BTC_KVB`, `SAT_VB`) is introduced in `policy/feerate.h` for feerate formatting.
     * The `CFeeRate::ToString()` method is updated to use `FeeRateFormat`.
     * All relevant function calls have been updated to use the new `FeeRateFormat` enum for formatting and `FeeEstimateMode` for fee estimation mode.

   This refactoring separates these unrelated responsibilities to improve code clarity.

ACKs for top commit:
  l0rinc:
    ACK c1355493e2
  furszy:
    utACK c1355493e2
  musaHaruna:
    ACK [c135549](c1355493e2) — reviewed in the context of PR [34075](https://github.com/bitcoin/bitcoin/pull/34075)
  willcl-ark:
    ACK c1355493e2

Tree-SHA512: 7cbe36350744313d3d688d3fd282a58c441af1818b1e8ad9cddbc911c499a5205f8d4a39c36b21fed60542db1ef763eb69752d141bcef3393bf33c0922018645

src/test/fuzz/string.cpp

@@ -22,6 +22,7 @@
 #include <test/fuzz/FuzzedDataProvider.h>
 #include <test/fuzz/fuzz.h>
 #include <test/fuzz/util.h>
+#include <util/fees.h>
 #include <util/strencodings.h>
 #include <util/string.h>
 #include <util/translation.h>
@@ -34,8 +35,6 @@
 #include <string>
 #include <vector>
 
-enum class FeeEstimateMode;
-
 using common::AmountErrMsg;
 using common::AmountHighWarn;
 using common::FeeModeFromString;