Implement fundrawtransaction JSON-RPC

[jrick]

This method is currently unimplemented, and should be implemented as part of the raw transaction JSON-RPC API.

The behavior should be similar to Bitcoin Core's implementation. Some differences that must be accounted for are:

• The funding account must be specified as a non-optional parameter
• Address types (for change) can likely be ignored, since we have a malleability fix for normal P2PKH
• We don't have separate watching-only addresses in wallets with other private keys, so includeWatching makes little sense
• RBF (BIP125) isn't implemented by dcrwallet and doesn't need support in this request
• There is no fee rate estimation, so the estimate_mode parameter doesn't make sense
• There is no segwit distinction in Decred, so the iswitness parameter is unnecessary

[vctt94]

Can I get this issue?