Camera: Skip non platform-specifc CameraFeed types in Linux/macOS driver

[j20001970]

Closes #100522

Platform implementations of CameraServer was expecting added camera feeds always being corresponding CameraFeed subclass, this breaks the possibility of implementing custom camera feeds with either GDScript or GDExtension.

This PR add type check, ignoring feeds that are not added by the server itself.

[akien-mga]

The changes seem fine, but what's the use case for having feeds registered in CameraServer that will only be skipped?

In other words, should these be silent skips like this, or ERR_FAIL_NULL checks?

[j20001970]

Considering a custom feed written in GDScript, it has to be registered in CameraServer, so that it can be referenced with feed id by its consumer (i.e. CameraTexture). When the server iterating over registered feeds, downcasting to platform-specific subclass without check leads to null reference if such custom feed is registered.

As for whether skipped feeds should be silent or error, since the intention of this PR is to enable custom feed in GDScript/GDExtension, I think they should stay silent.

[akien-mga]

Thanks!