Support getblocktemplate and submitwork

[jrawsthorne]

I've been messing around with some mining stuff. It would be useful to support getblocktemplate and submitwork without using them manually with client.call().

I'll try and add a pr that implements this, the only issue is figuring out the exact format of each field for getblocktemplate because of all the different bips

[0xB10C]

I'm picking getblocktemplate up with a focus on the "template" mode (Bitcoin Core returns a block template to the client). The "proposal" mode can be added in a follow-up if needed.