candidate-backing: modify logic to process Second message to support elastic scaling

[axaysagathiya]

Issue summary

To support elastic scaling, we need to make some minor changes in logic to process the Second overseer message in the candidate backing subsystem.

Implementation details

make sure the local validator is not disabled before processing the candidate.

Update the logic to ensure the candidate is from our assignment

• using the core index, validators can find parachains assigned to the core. (The claim queue will provide this info)
  We need to add a claim queue runtime method if we don't have it already.
• The candidate's parachain ID should be in the list of parachains we get from the claim queue.

Other information and links