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
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