feat: handle eth case in native erc20 token address endpoint

[juanbono]

What ❔

Changes the native ERC20 address endpoint (get_native_token_address) to return a 0 if there is no native ERC20 deployed. This means that the native token in the network is ETH.

Since we are relying on the native_erc20.json file for now to detect a native ERC20 token, it just returns 0 if the files does not exist. In the future, If we move to config file we can check the existence/contents of the env var.

Why ❔

This is mostly added for compatibility purposes.

How to test

Just remove the etc/tokens/native_erc20.json file and query the endpoint:

❯ curl -X  POST http://localhost:3050 -H "Content-Type: application/json" --data '{"jsonrpc":"2.0","method":"zks_getNativeTokenAddress","params":[""],"id":1}'
# expected
{"jsonrpc":"2.0","result":"0x0000000000000000000000000000000000000000","id":1}

Checklist

• PR title corresponds to the body of PR (we generate changelog entries from PRs).
• Tests for the changes have been added / updated.
• Documentation comments have been added / updated.
• Code has been formatted via zk fmt and zk lint.
• Spellcheck has been run via zk spellcheck.