addpsbtinput: New RPC command to add funds to a psbt

This is the sister command of addpsbtoutput.

Adds inputs equal to or greater than the amount requests, reservers them, and reports important information back out to the user.

Changelog-Added: New low-level RPC command addpsbtinput to fund PSBTs directly and help with complex splices & dual-opens.

common/jsonrpc_errors.h

@@ -68,6 +68,7 @@ enum jsonrpc_errcode {
 	FUNDING_UNKNOWN_CHANNEL = 311,
 	FUNDING_STATE_INVALID = 312,
 	FUND_CANNOT_AFFORD_WITH_EMERGENCY = 313,
+	FUND_INPUT_IS_ZERO = 314,
 
 	/* Splice errors */
 	SPLICE_BROADCAST_FAIL = 350,