CCIP2 Resolver Full Specs

[sshmatrix]

This is a place for organising all details of the CCIP2 Resolver for the final docs. @0xc0de4c0ffee: the older README version is perhaps slightly outdated[?] Can you start adding bullet points in the comments here? I will expand them in detail in the main body down below using stuff from older README. You'll be able to outline the resolver better than me. I'll do the same for the client part.

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<expand on bullets here>

[sshmatrix]

<add bullets here>

[0xc0de4c0ffee]

• default callback signed data format/prefix
  <recordhash.selector, signer, signature, result>
• Approved callback signed data format/prefix
  <approved.selector, signer, signer_signature, owner_signature, result>