This PR implements the block author API method. Runtimes ought to
implement it such that it corresponds to the `coinbase` EVM opcode.

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Signed-off-by: xermicus <cyrill@parity.io>
Signed-off-by: Cyrill Leutwiler <bigcyrill@hotmail.com>
Co-authored-by: command-bot <>
Co-authored-by: Alexander Theißen <alex.theissen@me.com>
Co-authored-by: cmd[bot] <41898282+github-actions[bot]@users.noreply.github.com>