EIP2474 - Coinbase calls

# Simple Summary

Allow contracts to be called directly by block.coinbase (block validator), without a transaction.

# Abstract

In proof-of-work blockchains, validators are known as miners.

The validator might want to execute functions directly, without having to sign a transaction. Some examples might be presenting a proof in a contract for an change which also benefits the validator.

A notable example would be when a validator want to act as an EIP-1077 Gas Relayer, incentivized to pick up fees from meta transactions. Without this change, they can do so by signing from any address a gasPrice = 0 transaction with the gas relayed call. However this brings an overhead of a signed transaction by validator that does nothing, as msg.sender is never used, and there is no gas cost to EVM charge.

This proposal makes possible to remove this unused ecrecover.

# Motivation

In order to reduce the overhead of calls that don't use msg.sender and are being called by validator with tx.gasPrice = 0.

# Specification

The calls to be executed by block.coinbase would be included first at block, and would consume normally the gas of block, however they won't pay/cost gas, instead the call logic would pay the validator in other form.

Would be valid to execute any calls without a transaction by the block coinbase, except when the validator call tries to read msg.sender, which would throw an invalid jump.

Calls included by the validator would have tx.origin = block.coinbase and gas.price = 0 for the rest of call stack, the rest follows as normal calls.

# Rationale

TBD

# Backwards Compatibility

tx.origin = block.coinbase could cause some issues on bad designed contracts, such as using tx.origin to validate a signature, an analysis on how contracts use tx.origin might be useful to decide if this is a good choice.

# Test Cases

TBD

# Implementation

TBD

# Security Considerations

TBD