inner representation of `ChainId` shouldn't be `String`

[tdelabro]

https://github.com/starkware-libs/starknet-api/blob/main/src/core.rs#L20-L21

here ChainId is represented as a String

But in the specs the ChainId is used to compute the tx hash of all the transactions. This means it should fit in a Felt meaning it should be at most a ShortString of max size 31 characters.

I recommend we store it as a Felt for its internal representation, as it will simplify its usage for tx hash computation, and implement Display on it in a way that returns the short string representation.

With the current inner repr as String and no new method to make sure the max length is respected, anybody can write code that will break the protocol.