Add spawn_dedicated_client to v2 group subscriber consumer config

[urmastalimaa]

When spawn_dedicated_client is set to {true, Bootstrap}, a separate client is spawned for each partition subscriber, allowing the subscribers themselves to block the fetch loop with setting min_bytes > 0, without affecting one another.

[urmastalimaa]

This does not work as desired, as the client is kept alive even when workers are terminated, e.g. when new assignments are received. See the PR comment for possible approaches

[urmastalimaa]

Options for dedicated clients in v2 group subscriber

Topic subscriber initiates a client itself

• Change brod_topic_subscriber to accept a bootstrap in addition to a client ID
• Start and link to the client in topic subscriber init

Pros: Relatively simple to implement
Cons: Significant change to topic subscriber start_link interface. When a group subscriber receives new assignments, all clients for the previously assigned partitions are disconnected and reconnected - a significant waste when assignment change.

v2 group subscriber maintains a pool of clients

• Add to v2 group subscriber state a pool of clients, where each client may be associated with a worker
• When starting a new worker, find an unassigned client from the pool or instantiate a new client
• Disassociate the client and worker when a worker terminates (either by seeing the EXIT or in terminate_worker)
• [Resource optimization] Disconnect a client when it has been unassigned for X time

Pros: Client connections remain intact when assignments change
Cons: Significant changes to v2 group subscriber internal logic