Backing: Prospective Parachains subsystem

[kishansagathiya]

Issue summary

• This is part of async backing.
• Asynchronous backing changes the runtime to accept parachain candidates from a certain allowed range of historic relay-parents. This means we can now build prospective parachains – that is, trees of potential (but likely) future parachain blocks. This is the subsystem responsible for doing so.
• Start with creating a skeleton PR that leads to subissues.

Other information and links

• https://paritytech.github.io/polkadot-sdk/book/node/backing/prospective-parachains.html#prospective-parachains
• https://github.com/paritytech/polkadot-sdk/blob/aa68ea58f389c2aa4eefab4bf7bc7b787dd56580/polkadot/node/core/prospective-parachains/

[kishansagathiya]

Requires skeleton and thus sub-issues. Estimating it at 8, until additional details and sub-issues arrives.