Extend the Gateway contract to store protocol contract addresses

[fadeev]

If we decide that the Gateway contract should stay minimal, then I propose we rename the current Gateway to GatewayInternal or something, and make the actual Gateway in front of it.

If we position the Gateway contract as the single entry point to ZetaChain, we need to make sure it can additional seemingly non-core functionality like storing contract addresses and reporting gas.

[lumtis]

Can you provide the extended interface you would consider for the gateway on top of withdraw/deposit/call methods?
I would personally avoid having several gateway contract types.

[fadeev]

@lumtis

_type would be zetaToken, tssUpdater or any other contract address:

function getAddress(string memory _type) external view returns (address);

This function is for when _type is zrc20, which also requires a symbol like "BTC":

function getAddress(string memory _type, string memory _symbol) external view returns (address);

[fadeev]

Maybe this can be implemented as a precompiled contract instead: #259