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/rpc/client.cpp

@@ -41,7 +41,8 @@ static const CRPCConvertParam vRPCConvertParams[] =
     { "sendtoaddress", 5 , "replaceable" },
     { "sendtoaddress", 6 , "conf_target" },
     { "sendtoaddress", 8, "avoid_reuse" },
-    { "sendtoaddress", 9, "verbose"},
+    { "sendtoaddress", 9, "fee_rate"},
+    { "sendtoaddress", 10, "verbose"},
     { "settxfee", 0, "amount" },
     { "sethdseed", 0, "newkeypool" },
     { "getreceivedbyaddress", 1, "minconf" },
@@ -73,7 +74,8 @@ static const CRPCConvertParam vRPCConvertParams[] =
     { "sendmany", 4, "subtractfeefrom" },
     { "sendmany", 5 , "replaceable" },
     { "sendmany", 6 , "conf_target" },
-    { "sendmany", 8, "verbose" },
+    { "sendmany", 8, "fee_rate"},
+    { "sendmany", 9, "verbose" },
     { "deriveaddresses", 1, "range" },
     { "scantxoutset", 1, "scanobjects" },
     { "addmultisigaddress", 0, "nrequired" },
@@ -129,7 +131,8 @@ static const CRPCConvertParam vRPCConvertParams[] =
     { "lockunspent", 1, "transactions" },
     { "send", 0, "outputs" },
     { "send", 1, "conf_target" },
-    { "send", 3, "options" },
+    { "send", 3, "fee_rate"},
+    { "send", 4, "options" },
     { "importprivkey", 2, "rescan" },
     { "importaddress", 2, "rescan" },
     { "importaddress", 3, "p2sh" },