Add channel based provider

[XAMPPRocky]

In order to make things like benchmarking easier, we should add a watch::channel based provider. This would make it easier to write tests and create benchmarks of the system handles large amounts of gameservers and filters without requiring a filesystem or a kubernetes installation.

It would also allow for unifying other providers. Currently both the Agones provider and file provider both handle applying changes slightly differently, but both of them could move to sending changes through the channel.

This wouldn't be a user (as in CLI user) visible provider, but it would be something for use in code, could even allow for others to easily add providers.