feat(proof): pathfinder_getClassProof endpoint

[odesenfans]

Added a new endpoint pathfinder/v0.1/pathfinder_getClassProof that returns the Merkle proof of any class hash in the class trie.

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This new endpoint is necessary to run the Starknet OS to completion as we need the full class commitment as input of the OS.

[sistemd]

I see nothing objectionable, but @kkovaacs is our trie expert.

Also not sure if something like this will be in the v0.8 spec maybe, so this endpoint won't be necessary anymore. There's a PR open for this in the specs repo. (I understand you may already be aware of this.)

[HermanObst]

Hey @kkovaacs! I addressed your comments and update to latest main.
Im getting a spurious clippy error, but the rest looks good :)

[HermanObst]

@kkovaacs sorry I had a misconfiguration locally, now everything should be fine :)

[kkovaacs]

LGTM, thanks a lot for this!

Also, please note that the JSON-RPC 0.8.0 specification will have this functionality as part of its starknet_getProof method.