feat(prospective-parachains) implement Fragment Chain

[EclesioMeloJunior]

Description

• Implement CandidateStorage, that stores all kinds of candidate without any restriction on whether or not candidates form a chain. definition

  • Use map[CandidateHash]any to represent HashSet<CandidateHash>
  • Implement possible_backed_para_children using iter.Seq[CandidateEntry]

• Implement subsystem_util/inclusion_emulator::Constraints definition. This is the actions that can be taken by a new parachain block. This information is needed for Scope struct under fragment chain.

  • Should be possible to create a inclusion_emulator::Constraints from polkadot/primitives/async_backing/v8::Constraints
    • Code for From<PrimitiveConstraints> for Constraints here.
    • Definition of v8::Constraints here

• Implement BackedChain definition, includes candidates pending availability and candidates which may be backed on chain. It will be used in the FragmentChain struct as the best chain of beckable candidates.

• Implement FragmentChain definition

• Include polkadot-sdk tests here

• Provide a better documentation