feat: Add native ERC20 token address endpoint

[juanbono]

What ❔

Adds a new RPC endpoint to get the Address of the native ERC20

it only adds a new method in the zks namespace.

Why ❔

This is needed by the explorer. More context here: #31

How to test

You first need to run zk init --native-erc20 to deploy the required contracts and generate the native_erc20.json file.
Then run the following command:

$ curl -X POST localhost:3050 -H "Content-Type: application/json" --data '{"jsonrpc":"2.0", "method":"zks_getNativeTokenAddress", "params":[], "id":1}'

# expected output:
{"jsonrpc":"2.0","result":"0x2be576fb1996aeca500460c6691ce594a0aa98ae","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 cargo spellcheck --cfg=./spellcheck/era.cfg --code 1.

[fkrause98]

This endpoint fails for me with this curl:

curl -X  http://localhost:3050 \
             -H "Content-Type: application/json" \
              --data '{"jsonrpc":"2.0","method":"zks_getNativeTokenAddress","params":[""],"id":1}'

I get this message: curl: (52) Empty reply from server.
And the server prints this error:

thread 'jsonrpsee-http-worker' panicked at /Users/fran/Programming/Zksync/zksync-era/core/lib/zksync_core/src/api_server/web3/backend_jsonrpsee/namespaces/zks.rs:42:9:
not yet implemented

Let me know if I'm testing this wrong, though

EDIT:
Tested again, works great with the command above:

curl -X  http://localhost:3050 \
             -H "Content-Type: application/json" \
              --data '{"jsonrpc":"2.0","method":"zks_getNativeTokenAddress","params":[""],"id":1}'
{"jsonrpc":"2.0","result":"0x2be576fb1996aeca500460c6691ce594a0aa98ae","id":1}⏎