Update organization cluster agents query to accept ALL filter

MR: Add `ALL` filter to cluster agent query (!192329 - merged)

Description

Based on the design and discussions in New UI in the admin section to support mapping ... (#513370 - closed), the organization cluster agents table within the admin settings will be paginated. If we were to use our existing filters we would need to query with the AVAILABLE and UNMAPPED filters, which returns separate pagination cursors. This would result in the frontend having to do the bulk of the work by reconciling the two results and also a poor user experience when an agent goes from mapped to unmapped and vice versa.

We can instead introduce a new ALL filter that returns all agents for the given namespace.

Acceptance criteria

• Cluster agent query now accepts an ALL filter, which returns all cluster agents for the given namespace.