Add block template retrieval feature

[tigeryant]

This PR adds support for block template retrieval via the IPC interface. Obtaining block templates from a node is an important part of the mining process. Miners can add their own transactions before mining and submitting the block. An example is added to the README to show usage in a downstream repository.

[pseudoramdom]

This is great! Thanks for the pull request.
Currently the block_template has only get_block_template.
Do you think it'll be better to namespace it under MiningInterface? That way I can add other functions like create_block as part of the mining interface

[tigeryant]

Thanks for the feedback. I've addressed this in these commits. Please review again.