Extend APIServerSource to watch multiple namespaces

[gab-satchi]

Problem
There's a bit of overhead if a user wishes to watch multiple namespaces for api server events. They currently need to create an APIServerSource for each of the namespaces they wish to watch.

This issue is to explore a solution where multiple namespaces can be watched with a single APIServerSource. There was an issue about watching a different namespace but this issue extends that a step further.

One possible solution would be to introduce a label selector to the api. If it's not set, it could default to just watching the namespace the source resides in to maintain backwards compatibility. An empty selector can mean all namespaces. This would be similar to how the sugar controller configuration is done.

Persona:
Which persona is this feature for?
System integrator

Additional context (optional)
The permissions checker and the receiver adapter will need to iterate over all namespaces

[pierDipi]

I don't have concerns for the feature itself, one question below

One possible solution would be to introduce a label selector to the api. If it's not set, it could default to just watching the namespace the source resides in to maintain backwards compatibility. An empty selector can mean all namespaces. This would be similar to how the sugar controller configuration is done.

What all selected namespaces would mean? at the creation time? or are we planning to watch namespaces as they are created or deleted to dynamically watch resources in selected namespaces?

[gab-satchi]

I think this problem would exist even when a selector is used. The namespaces targeted can change dynamically.

I was thinking about the FilteredGlobalResync but I don't know if that could be used. When a label is removed from a namespace, it can't tell which api sources used to watch the namespace. I think a GlobalResync might be the most thorough option. So whenever a namespace event happens, we enqueue all the api sources.