Design Updates for Candidate backing subsystem to support Async backing + Elastic Scaling

[axaysagathiya]

Issue summary

All the design update issues to update the Candidate Backing Subsystem to support Async backing + Elastic scaling are listed in #3820

Other information and links

Acceptance criteria

• changes specification should be written (or written within issues)
• Estimable issues for changes that includes Async backing for Candidate Backing should be created

[axaysagathiya]

We need to remove the usage of Prospective parachain mode from the candidate backing subsystem.
As the perLeaf field of the CandidateBacking struct keeps a record of whether the active leaf has perspective parachain mode enabled or disabled, perLeaf will be removed in the future. Once we remove all the usage of this field.

[P1sar]

Issues that Axay have created during design

• candidate-backing-subsystem: Modify logic to handle ActiveLeavesUpdate to support elastic scaling #4327
• Update GetBackableCandidates struct and logic to process it #4296
• update CanSecond message process logic to support elastic scaling #4329
• find core index of candidate parachain using the validity statement #4324
• Change type of group id from ParaID to CoreIndex #4316

[axaysagathiya]

There are minor changes in the logic to process the validatedCandidateCommand::Second message. Raised an issue for the same #4402

[axaysagathiya]

I raised issues for modifying the logic to process the Second overseer message and Statement overseer message