feat(parachain): create skeleton for availability distribution subsystem

[haikoschol]

Issue summary

Create skeleton code for the implementation of the Availability Distribution subsystem which is described in the Host Implementers' Guide, utilizing the subsystem pattern that has been used in other Gossamer subsystems.

• Create a new package called availabilitydistribution in dot/parachain/availability-distribution
• Create a struct called AvailabilityDistribution and implement the Subsystem interface on it
• Create empty handlers for
  • parachain.ChunkFetchingRequest
  • parachain.PoVFetchingRequest
  • parachaintypes.AvailabilityDistributionMessageFetchPoV
  • parachaintypes.ActiveLeavesUpdateSignal
• Register availability distribution subsystem with overseer
• Forward the above messages to the availability distribution subsystem from overseer
• Define BlockState interface and add it to subsystem state (see section below for details)
• Add tests

Subsystem State

Most of the subsystem state should probably be covered when implementing message handling logic instead of in the skeleton. One exception is defining an interface for accessing runtime instances and ancestors of active leaves in the block state.

package availabilitydistribution

type BlockState interface {
    GetHeader(hash common.Hash) (*types.Header, error)
    GetRuntime(hash common.Hash) (instance runtime.Instance, err error)
}

type AvailabilityDistribution struct {
    blockState BlockState
    // ...
}

Other information and links

Look at other skeleton PRs to get an idea of what to code and what will become and TODO and a new issue

• feat(dot/parachain): Implement candidate backing skeleton #3497
• feat(parachain): availability store skeleton #3521
• collator protocol skeleton #3512
• availability distribution design document
• Parity node implementation

[ramiroJCB]

Hello guys I will be doing this one :)