wallet: introduce fee_rate (sat/vB) param/option

Create a fee_rate (sat/vB) RPC param and replace overloading the conf_target and
estimate_mode params in the following 6 RPCs with it:

- sendtoaddress
- sendmany
- send
- fundrawtransaction
- walletcreatefundedpsbt
- bumpfee

In RPC bumpfee, the previously existing fee_rate remains but the unit is changed
from BTC/kvB to sat/vB. This is a breaking change, but it should not be an
overly risky one, as the units change by a factor of 1e5 and any fees specified
in BTC/kvB after this commit will either be too low and raise an error or be 1
sat/vB and can be RBFed.

Update the test coverage for each RPC.

Co-authored-by: Murch <murch@murch.one>

src/util/fees.cpp

@@ -40,8 +40,6 @@ const std::vector<std::pair<std::string, FeeEstimateMode>>& FeeModeMap()
         {"unset", FeeEstimateMode::UNSET},
         {"economical", FeeEstimateMode::ECONOMICAL},
         {"conservative", FeeEstimateMode::CONSERVATIVE},
-        {(CURRENCY_UNIT + "/kB"), FeeEstimateMode::BTC_KB},
-        {(CURRENCY_ATOM + "/B"), FeeEstimateMode::SAT_B},
     };
     return FEE_MODES;
 }