Implement spendoutputs JSON-RPC method

[jrick]

This method provides greater coin control when creating and sending transactions. Rather than the wallet performing UTXO selection on its own, only the specified inputs and outputs (with the exception of wallet change) will be used.

This is intended to be a more ergonomic and easier to use alternative to the raw transaction RPCs (createrawtransaction, signrawtransaction, and sendrawtransaction).

[matthawkins90]

Conceptually, I'm super excited about this. Dealing with raw transactions manually is a pain.

[jrick]

An example on how this can be used:

$ cat spendoutputs.sh                                                                                                                 
#!/bin/ksh

exec ./dcrctl --testnet --wallet spendoutputs default - - <<EOF
$(jq -M -c < spendoutputs-previousoutpoints.txt)
$(jq -M -c < spendoutputs-outputs.txt)
EOF
$ cat spendoutputs-previousoutpoints.txt                                                                                              
[
  "952d607bfabf524bef523ec08bb743ede8618864fe8ae1418d6020286150dfc9:4",
  "c90939f5e0eeab4cb296c84b6883fa692892b60c0e0e60bec1cf0193e1372c3c:3"
]
$ cat spendoutputs-outputs.txt                                                                                                        
[
  {"address": "TsXVzz2xrAxEMUYGj7wRfFvEnEqsTJ2RARB", "amount": 1.23},
  {"address": "TsYFYPAJaApn2pGEkziHJ9VuHuhXWoRLGKw", "amount": 4.56}
]
$ ./spendoutputs.sh
e0456b1f52bc8db0ceab33fe34cb9206a9f685349a9fff2f8ea8291ce16eb891