[Feature Request]: allow subscribing to new blocks

[SuperFluffy]

Implementation ideas

A nice feature would be to subscribe to new blocks under a target namespace, maybe something like blob.Subscribe that takes 1 or more namespaces. It would be great to receive the current height and a map of namespace -> [blob] (the RPC should return an empty map and/or list if no blocks were written).

At the moment we can use header.Subscribe and then extract the most recent height. We can then follow this up with blob.GetAll using height and a single [MyNamespace] as parameters.

This is fine and not too inconvient, but a direct subscription to blocks that avoids the extra RPC would be a good feature for a data availability layer.

[Wondertan]

Thanks for the issue! We had something like this in mind for a while but never received any feature requests from any of teams. We should def do blob.Subsribe