feat: Dynamic subscribe/unsubscribe mechanism (or similar)

[chair28980]

With autosharding, nodes' shard subscriptions (relay) are influenced by:

• initial static configuration (pubsub topic or content topics hashed to pubsub topics)
• runtime, dynamic subscription to content topics

A useful feature may be a bandwidth management mechanism for relay nodes to choose dynamically the number of shards they support based on bandwidth availability. For example, when the network launches, it's reasonable for relay nodes to support all shards by default and gradually unsubscribe from shards as bandwidth increases, always preferring shards for which content topic subscriptions exist. If a node is configured with very restricted bandwidth, the relay subscriptions may even "convert" to a filter subscription, allowing the node to opt out of relay automatically.

Priority: Not critical for launch

Update: descoped from waku-org/pm#117, reserved for future work