[Ethereum]: Support obtaining the function signature from an Ethereum function ABI

[10gic]

Description

The function signature is useful for indicating the intended contract call and for computing the 4-byte selector.

There is a similar function, tw_ethereum_abi_function_get_signature, in the code; however, it is not exported, and its input argument is difficult to construct. Therefore, I added a new function named EthereumAbi.getFunctionSignature. This new function accepts an ABI JSON string as an argument, making it very easy to use.

How to test

Run Rust, C++, iOS, Android tests

Types of changes

Checklist

• Create pull request as draft initially, unless its complete.
• Add tests to cover changes as needed.
• Update documentation as needed.
• If there is a related Issue, mention it in the description.

If you're adding a new blockchain

• I have read the guidelines for adding a new blockchain.