test: add SSZ test vectors

[unnawut]

What was wrong?

Implement leanEthereum/leanSpec#380

• Removed TreeHash derive from SignedAttestation because it's not used and it's in turn requiring Signature to implement TreeHash as well which requires custom logic but also not used.
• Change Signature::inner from FixedBytes<SIGNATURE_SIZE> to LeanSigSignature so that the final Signature scheme type is used directly without needing FixedBytes in-between
• Implemented Encode/Decode on Signature replacing the FixedBytes implementation
• Because leanSpec is giving us the expected outputs in json and camelCase format, we have to parse it to Ream/Rust types. So I created passthrough_conversion, simple_conversion, nested_conversion, custom_conversion to handle the json camelCase conversion to Ream types.

To-Do

• I have read CONTRIBUTING.md.
• This PR title follows conventional commits.

[unnawut]

I need to do another round of self-review before marking this ready for review

[unnawut]

Not sure if this is the right way to do it, but since FixedBytes is how Ream stores it internally and needs conversion to LeanSigSignature to do any operation anyway, so I'm thinking we could just remove this interim FixedBytes and use LeanSigSignature directly

[KolbyML]

Will LeanSigSignature encode to FixedBytes though is the question? I will read your code, but that would be the concern