Add SPIRE Controller Manager configuration Section

[maia-iyer]

Part of #529

It makes sense that CRD APIs are only enabled when Tornjak is configured to do so (or when the SPIRE Controller manager is definitely deployed).

For now, we will trigger this behavior through an additional plugin configuration section in Tornjak:

plugins {
  ...

  # [optional] if this section is included, CRD APIs will work
  # /api/v1/spire/federations APIs will log warnings
  # if this section is excluded, CRD APIs will return errors
  # /api/v1/spire/federations APIs will work
  SPIRECRDManager {
    # [required] Tornjak will create CRDs with this label for the controller-manager to pick up
    class_name = "spire-mgmt-spire"
  }
  ...
}