[rpc] add new submitpackage RPC

It could be unsafe/confusing to create an actual mainnet interface while
package relay doesn't exist. However, a regtest-only interface allows
wallet/application devs to test current package policies.

src/rpc/client.cpp

@@ -110,6 +110,7 @@ static const CRPCConvertParam vRPCConvertParams[] =
     { "sendrawtransaction", 1, "maxfeerate" },
     { "testmempoolaccept", 0, "rawtxs" },
     { "testmempoolaccept", 1, "maxfeerate" },
+    { "submitpackage", 0, "package" },
     { "combinerawtransaction", 0, "txs" },
     { "fundrawtransaction", 1, "options" },
     { "fundrawtransaction", 2, "iswitness" },