feat(dot/parachain/backing): implement a func to get attested candidate from statement table

[axaysagathiya]

Description

• The statement table stores statements about candidates(parachain blocks) issued by authorities(parachain validators)
• to call methods of statement table, we have an interface with a few methods, Which I am implementing one by one.
• to know more about the statement table, read the description of issue feat(dot/parachain/backing): Implement Statement Table #3471
  • also, you can see it has a bunch of sub-issues, and this PR addresses one of them.

This PR implements a method for getting an attested candidate from the statement table for the candidate hash.

Changes

• Added extra argument to interface method to get an attested candidate
• Implemented the same method of an interface
• There is no need for scale encoding/decoding to attestedCandidate, so I removed scale tags from the same struct.
• wrote unit tests for the same

Rust implementation: https://github.com/paritytech/polkadot-sdk/blob/7ca0d65f19497ac1c3c7ad6315f1a0acb2ca32f8/polkadot/statement-table/src/generic.rs#L318-L334

Issues

#3471

Primary Reviewer

@kishansagathiya @edwardmack

[edwardmack]

Great work.