feat(parachain/availability-distribution): implement fetchChunkTask

[haikoschol]

Issue summary

This functionality is part of the message handling logic for ActiveLeavesUpdateSignal.

Implementation details

Implement the type fetchChunkTask (or whatever name you prefer) for fetching erasure chunks from validators and keeping track of leaves for which those chunks are relevant.

Apart from a slice of hashes representing the relevant leaves, fetchChunkTask needs to have access to the overseer channel for sending request messages to the network bridge.

Other information and links

• Parity node implementation

Acceptance criteria

• New code is 60% covered with unit tests