feat: add `eth_chain_id` entrypoint

[enitrat]

• Add an eth_chain_id entrypoint to the KakarotCore contract
• This entrypoint will return the chain_id field of the tx_info, modulo 2**53
• Replace all references to tx_info.chain_id by this value returned from the Kakarot Contract

Closes #920

[bitfalt]

hii! Is this issue open for contributors? If so I'd love to tackle this issue. I assume I would need to use get_tx_info().unbox() and then return the chain_id field after applying modulo 2**53, then refactor the code with the value returned from it.

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[enitrat]

Hi! Yes, you can do it.

To help you, here's an existing implementation in old cairo.

https://github.com/kkrt-labs/kakarot/blob/7a83ddf6cfb7a69d995245291fe11c1d2444e408/src/kakarot/library.cairo#L124-L131

You could generally just find and replace all occurences of use of this tx_info.chain_id by a call to this function

[bitfalt]

Okay! Thank you so much for providing a link to the existing implementation, will start ASAP!