feat(dot/parachain): create skeleton for Availability Recovery subsystem

[edwardmack]

Issue summary

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

• Create new package called availabilityrecovery
• Create struct to define RecoveryStrategyKind which defines the PoV recovery strategy to use. for example
• Create struct to define AvailabilityRecovorySubsystem, see for example and implement Subsystem interface for it.
• Create struct to define State, see for example
• Create (empty) methods to handle the following messages:
  • RecoverAvailableData
  • overseer signal: ActiveLeavesUpdate
• Forward above messages to the availability recovery subsystem from the overseer
• Create unit tests for implemented code.

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