Use ConfigCat to throttle adoption of PVC

[kylos101]

Is your feature request related to a problem? Please describe

Presently, we rely on a few things to determine if a workspace should start with PVC.

1. They are restoring a workspace that previously used PVC (either from a prebuild or prior run of a regular workspace).
2. A new workspace is being created, the user has the persistent_volume_claim feature flag enabled, while also being enabled for workspace classes.
3. A new workspace is being created from the context of a prebuild workspace that was run with PVC (which can happen at least two ways, 1. the project has PVC enabled or 2. the prebuild workspace has the persistent_volume_claim feature flag persisted)

Describe the behaviour you'd like

For Case 2 (above), create a new configcat setting, and add a rule that uses Target % of users, which'll control the % of regular workspace starts that include the persistent_volume_claim feature flag. From a default perspective, for self-hosted, I think we'll want the new config setting to default to false, at least for now.

Case 1 should not need any changes. Case 3 is already covered by persistent_volume_claim (Prebuilds with Persistent Volume Claim) in configcat.

Then, in configcat, for both rules, we could be flexible with how we roll this out.

Internal conversation about how to configure rules in ConfigCat.

[kylos101]

Related: #12840