Feat: add utility function check for ERC-20 token allowance

[GideonBature]

This utility function interacts with the blockchain to call the allowance function which returns the price/value/number of allowance. Also, there are unit tests covering several test-cases.

Please do have a look at it, to run the test cases one will need the token address as well as the rpc provider which the token was deployed on. Thank you!

ref: Connect to an Ethereum Node (ethers_rs docs)

[Abeeujah]

Hi @GideonBature Great job so far, but a bit of misunderstanding I see... This function is to read allowance for ERC20 tokens deployed on Starknet, not on Ethereum.

[GideonBature]

Oh... 😅 I see, I misunderstood it then. I will make necessary changes and push again. Thank you for the review!

[GideonBature]

@Abeeujah Please do check. I have re-implemented it for Starknet Blockchain.

[Abeeujah]

Hi @GideonBature,

Thank you for your submission, here's a few suggestions:

1. Environment Variables: To improve security and configuration management, I'd recommend you refactor the code to retrieve the RPC_URL and TOKEN_CONTRACT_ADDRESS values from environment variables. Please also ensure that the .env.sample file is updated to reflect these changes.

2. Test Coverage: While your test cases address certain scenarios, it's crucial to include a positive or "success" test case to validate the expected behavior of the code under normal conditions. This will provide a more comprehensive assessment of its functionality.

3. Test Organization: For better code organization and maintainability, I suggest relocating the test cases from their current inline location to the dedicated tests folder. This separation of concerns will improve the overall structure of the project and make it easier to manage and expand the test suite in the future.

[GideonBature]

Noted, I will do all these. For the tests, I thought about that, but seeing that the issue said unit tests, that was why I put it in same directory with the function. But I will do this. Thank you so much for these reviews. I will implement these and push again.

[GideonBature]

Please do check if it's okay now, for the TEST_TOKEN, because I will have to convert it to Felt while testing using the felt!(" ") macros, I can't use a variable, the macros only accepts a &str as it needs to know the exact size/length of what it's working with.