Add "filter" functionality for our primary views

Similar to https://github.com/confluentinc/vscode/issues/496, we could add a "filter" action on (Resources, Topics, Schemas) views with many items. This would likely involve:
- prompting the user with an input box
- saving the provided filter value to workspace state (e.g. `.filter`)
- refreshing the associated view
- looking up the filter within the view's `getChildren()` method and only returning items matching the filter

This would also require a "clear filter" command to restore full visibility to the view.