Add optional filter to subscription reconciler

[ryannedolan]

Summary

Added optional filter predicate to subscription reconciler.

Details

One way to serve multiple use-cases is to run Hoptimator in multiple namespaces, each with different configurations. However, this proves unwieldy in practice, because each namespace has its own operational overhead (e.g. RBAC policies). Instead, it is convenient to have Hoptimator partition a single namespace based on its own criteria (e.g. subscription prefix).

To that end, we add support for filtering Subscriptions. This enables an operator to spin up multiple Subscription controllers, each with different filters and different configurations -- all in the same namespace. For example, an operator may want to treat Subscriptions differently if they have a -staging or -prod suffix.

Testing

Tested in downstream project.