RPC and ZeroMQ APIs to support p2pool

Adds the following:

- "get_miner_data" to RPC API
- "json-miner-data" to ZeroMQ subscriber contexts

Both provide the necessary data to create a custom block template. They are used by p2pool.

Data provided:

- major fork version
- current height
- previous block id
- RandomX seed hash
- network difficulty
- median block weight
- coins mined by the network so far
- mineable mempool transactions

docs/ZMQ.md

@@ -25,6 +25,8 @@ allows for filtering on: (1) format, (2) context, and (3) event.
      Includes previously unseen transactions in a block but _not_ the
      `miner_tx`. Does not "re-publish" after a reorg. Includes `do_not_relay`
      transactions.
+   * `miner_data` - provides the necessary data to create a custom block template
+     Available only in the `full` context.
 
 The subscription topics are formatted as `format-context-event`, with prefix
 matching supported by both Monero and ZMQ. The `format`, `context` and `event`