[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.

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Signed-off-by: xermicus <cyrill@parity.io>
Signed-off-by: Cyrill Leutwiler <bigcyrill@hotmail.com>
Co-authored-by: command-bot <>