[pallet-revive] implement the base fee API (#6964)
This PR implements the base fee syscall API method. Currently this is implemented as a compile time constant in the revive compiler, returning 0. However, since this is an opocde, if we ever need to implement it for compatibility reasons with [EIP-1559](https://github.com/ethereum/EIPs/blob/master/EIPS/eip-1559.md), it would break already deployed contracts. Thus we provide a syscall method instead. --------- Signed-off-by: xermicus <[email protected]> Signed-off-by: Cyrill Leutwiler <[email protected]> Co-authored-by: command-bot <>