Handle ZRC-20 tokens

[fadeev]

Right now localnet depends on test contract for ZRC-20 tokens:

localnet/packages/localnet/src/index.ts

Line 269 in 30d4f20

testContracts.zrc20.target,

The idea is to be able to use localnet not with a specific test contract, but with any example contract.

Ideally, the localnet environment simulates the real testnet and allows the gateway to accept ERC-20 tokens, and mints ZRC-20 tokens (like ZetaChain would) and sends them to a universal contract.

I think we can deploy a test ERC-20 as part of protocol contracts in localnet and just handle minting of ZRC-20s.

It should also be possible to send ETH and get ZRC-20 ETH.

The test case can be an example contract here:

zeta-chain/example-contracts#189

[fadeev]

@skosito