rpc: implement eth_coinbase

[fedekunze]

Description

Implements eth_coinbase JSON-RPC query with returns the block proposer's validator address in hex format.

test:

=== RUN   TestEth_coinbase
    TestEth_coinbase: rpc_test.go:150: Got coinbase block proposer: 0x1364216b0243482d8a2dac685410093d35215960
--- PASS: TestEth_coinbase (1.01s)

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[noot]

this looks like it returns the coinbase address on the current block, not the coinbase address of this node?
https://github.com/ethereum/wiki/wiki/JSON-RPC#eth_coinbase

not sure if this is functionally correct, what is this used for?

[fedekunze]

I updated it to return the validator address of the node 👍

[noot]

nice :D