feat(parachain/availability-distribution): handle ChunkFetchingRequest

[haikoschol]

Issue summary

This is a network message received from other validators. The node responds with an erasure chunk from the local availability store.

Implementation details

When receiving a request, send availabilitystore.QueryChunk to the availability store via the overseer, passing the candidate hash and validator index from the request. Receive the response from the channel included in QueryChunk, construct a ChunkFetchingResponse from it and send it to the requesting peer.

Protocol versions v1 and v2 exist for chunk fetching. The ChunkFetchingRequest message is structurally identical in both versions. There is a subtle difference in the meaning of validator index between the two request messages, but it should not affect this issue. The ChunkFetchingResponse message in v1 omits the chunks index. In v2 it contains the full ErasureChunk object.
Our implementation only covers v1 so far. We should probably add the chunk index to this struct and only support v2 for the initial
implementation, analogous the approach in other subsystems.

The implementation depends on support for receiving requests of request/response network protocols being implemented. This is tracked in #4453.

Other information and links

• Parity node implementation

Acceptance criteria

• New code is 60% covered with unit tests