add /purchaseticket API endpoint

[itswisdomagain]

Ticket purchase feature without user signup/login can be accessed by making a POST request to api/{version}/purchaseticket where version can be v1 or v2, passing UserPubKeyAddr in post body.

A pubkeyaddr can be generated by executing the following dcrctl commands against dcrwallet.
First, create a new wallet address with getnewaddress. Then, call validateaddress and retrieve the address listed in the pubkeyaddr field of the response. The pubkeyaddr starts with Tk.

The following is an example:

$ dcrctl --testnet --wallet getnewaddress
TsExampleAddr1For2Demo3PurposesOnly
$ dcrctl --testnet --wallet validateaddress TsExampleAddr1For2Demo3PurposesOnly
{
  "isvalid": true,
  "address": "TsExampleAddr1For2Demo3PurposesOnly",
  "ismine": true,
  "pubkeyaddr": "TkExample0Addr1For2Demo4Purposes5Only6Do7Not8Use9Pls0",
  "pubkey": "022801337beefc0ffee1dab8d4ffa898a782466c9a1fc00ca8863de5438dc07dcc",
  "iscompressed": true,
  "account": "default"
}

Calling the new endpoint with a valid pubkeyaddr returns the following (sample) data:

{
    "status": "success",
    "code": 0,
    "message": "APIPurchaseTicket: purchaseinfo retrieved for existing userPubKeyAddr",
    "data": {
        "PoolAddress": "TsoWCv4btvs2276yZ9qQkNL1xv5EbrBLjAJ",
        "PoolFees": 7.5,
        "Script": "512102ffbb68c445608d24239c4f1435cea18a00a652d5296481123bbc40546f01415521038d980deefd1d7bee8e195204b2e00f87904ca992bf7995bca56e928c8fefc8b552ae",
        "TicketAddress": "TcZJ5jMFBbekWAueyAmhyb5SjUM5kw5wbdU",
        "VoteBits": 1,
        "VoteBitsVersion": 0
    }
}

Complete the ticket purchase using the data above:

• Import the redeem script:

$ dcrctl --testnet --wallet importscript 512102ffbb68c445608d24239c4f1435cea18a00a652d5296481123bbc40546f01415521038d980deefd1d7bee8e195204b2e00f87904ca992bf7995bca56e928c8fefc8b552ae

• Purchase the ticket using the ticket address, fee address and fee percentage values. The dcrctl command usage is purchaseticket "fromaccount" spendlimit (minconf=1 "ticketaddress" numtickets "pooladdress" poolfees expiry "comment" ticketfee) (see the doc for details)

$ dcrctl --testnet --wallet purchaseticket "default" 100 1 TcZJ5jMFBbekWAueyAmhyb5SjUM5kw5wbdU 1 TsoWCv4btvs2276yZ9qQkNL1xv5EbrBLjAJ 7.5