Availability Subsystems: availability distribution subsystem

[kishansagathiya]

Issue summary

This subsystem is responsible for distribution availability data to peers. Availability data are chunks, PoVs and AvailableData (which is PoV + PersistedValidationData).

Sub Issues

• Availability store #3415
• Implement availability data pruning in availability store #3561
• feat(dot/parachain): Implement PoV Distributor #3275
• Issue for chunk requestor https://paritytech.github.io/polkadot-sdk/book/node/availability/availability-distribution.html#chunk-requester
• feat(dot/parachain/availability): process overseer signals for availability distribution #3645
• feat(parachain): create skeleton for availability distribution subsystem #4485
• feat(parachain/availability-distribution): handle ChunkFetchingRequest #4487
• feat(parachain/availability-distribution): handle PoVFetchingRequest #4488
• feat(parachain/availability-distribution): handle AvailabilityDistributionMessageFetchPoV #4489
• feat(parachain/availability-distribution): implement get_block_ancestors_in_same_session #4490
• feat(parachain/availability-distribution): implement fetchChunkTask #4491
• feat(parachain/availability-distribution): implement management of chunk fetching tasks #4492
• feat(parachain/availability-distribution): implement cache for runtime info #4494

Other information and links

Code link in polkadot sdk https://github.com/paritytech/polkadot-sdk/blob/7035034710ecb9c6a786284e5f771364c520598d/polkadot/node/network/availability-distribution/src/lib.rs#L86

Node Implementor's guide https://paritytech.github.io/polkadot-sdk/book/node/availability/availability-distribution.html

[kishansagathiya]

Considering the size of work and sub-issues, plus since it is a new work, there are chances for finding new unknowns (mostly smaller unknows), estimating it at 8.