ERC20 GET balance should return just ERC20 balance

[moisses89]

What is needed?

The current implementation is adding always the Native token balance when we get the erc20 balance. https://github.com/safe-global/safe-eth-py/blob/72b7ed6daded3aa98cd536eb157551365f903a09/gnosis/eth/ethereum_client.py#L541

Possible solutions

• Remove the Native token balance from the response of Erc20Manager.get_balance()
• Add an optional parameter setting if we client wants to include the Native token balance or not.