Implement eth_get_balance

[saimeunt]

This PR implements eth_get_balance and use it instead of querying the balanceOf function of the native_token in the codebase. It also fixes a suspected bug in get_starknet_address from KakarotCore.

Pull Request type

Please check the type of change your PR introduces:

• Bugfix
• Feature
• Code style update (formatting, renaming)
• Refactoring (no functional changes, no API changes)
• Build-related changes
• Documentation content changes
• Other (please describe):

What is the current behavior?

The eth_get_balance is not implemented and thus we need to query the native_token directly in several parts of the codebase.

Resolves: #941

What is the new behavior?

The eth_get_balance is now implemented and replicates its eth_getBalance counterpart in the EVM.

• The implementation of eth_get_balance leverages the get_starknet_address function from KakarotCore to get the corresponding starknet address from an evm address. Once we have the underlying starknet address, we query the native_token balance. The function has also been added to ExtendedKakarotCore interface.
• When reading about the description of the get_starknet_address, I suspected a bug and from my understanding we should return the registered starknet address only if it is found and thus non-zero, so I think the if condition should be inversed, this is why my PR introduces a bugfix.
• The occurences of direct native_token querying have been replaced by eth_get_balance, in particular for the eth_validate_tx test helper I had to mock additional calls so account.get_evm_address returns the expected account_starknet_address.
• A simple test has been added to ensure the correct behavior of eth_get_balance both for uninitialized and initialized accounts.

Does this introduce a breaking change?

• Yes
• No

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This change is 

[enitrat]

great work!

[saimeunt]

@enitrat Thank you for your quick review and your Cairo tips, I made the requested changes by following your best practices insights.

I will apply to #991 as I'd like to continue my work on Kakarot so this is appreciated! 🙌