Add RPC "simulate" to execute a dry run of a transaction (#5069)

- Spec: https://github.com/XRPLF/XRPL-Standards/tree/master/XLS-0069d-simulate
- Also update signing methods to autofill fees better and properly handle transactions that require a non-standard fee.

include/xrpl/protocol/ErrorCodes.h

@@ -154,7 +154,10 @@ enum error_code_i {
     // deposit_authorized + credentials
     rpcBAD_CREDENTIALS = 96,
 
-    rpcLAST = rpcBAD_CREDENTIALS  // rpcLAST should always equal the last code.
+    // Simulate
+    rpcTX_SIGNED = 97,
+
+    rpcLAST = rpcTX_SIGNED  // rpcLAST should always equal the last code.
 };
 
 /** Codes returned in the `warnings` array of certain RPC commands.

include/xrpl/protocol/jss.h

@@ -115,6 +115,8 @@ JSS(SettleDelay);              // in: TransactionSign
 JSS(SendMax);                  // in: TransactionSign
 JSS(Sequence);                 // in/out: TransactionSign; field.
 JSS(SetFlag);                  // field.
+JSS(Signer);                   // field.
+JSS(Signers);                  // field.
 JSS(HookStateData);            // field.
 JSS(HookStateKey);             // field.
 JSS(SigningPubKey);            // field.