feat(parachain/collator): handle Collator's Declare message on the Validator side

[kishansagathiya]

Issue summary

• This is part of collator protocol.

• Over the collator protocol, you could receive a Declare message from a peer, which tells us that that peer is a collator.
  On receiving this message, we should verify its signature and add it in our list of collators.

• Should be done after collator protocol skeleton #3512 is merged.

• This message transfer is between two subsystems on a single node, so we have freedom on its structure.

• I have WIP code written for this, but not fully ready. Let me know if you want to take this up.

Other information and links

• Code reference in polkadot sdk
  https://github.com/paritytech/polkadot-sdk/blob/f4827dcd16fc5585a8d15c6a32bc27ecde00e7f9/polkadot/node/network/collator-protocol/src/validator_side/mod.rs#L760-L761

Make sure you follow the polkadot-sdk repo. The code has changed enough from the archived polkadot repo.

[kishansagathiya]

closed in #3529