Allow dynamic clients to be used in downstream OIDC flows

This is only a first commit towards making this feature work.
- Hook dynamic clients into fosite by returning them from the storage
  interface (after finding and validating them)
- In the auth endpoint, prevent the use of the username and password
  headers for dynamic clients to force them to use the browser-based
  login flows for all the upstream types
- Add happy path integration tests in supervisor_login_test.go
- Add lots of comments (and some small refactors) in
  supervisor_login_test.go to make it much easier to understand
- Add lots of unit tests for the auth endpoint regarding dynamic clients
  (more unit tests to be added for other endpoints in follow-up commits)
- Enhance crud.go to make lifetime=0 mean never garbage collect,
  since we want client secret storage Secrets to last forever
- Move the OIDCClient validation code to a package where it can be
  shared between the controller and the fosite storage interface
- Make shared test helpers for tests that need to create OIDC client
  secret storage Secrets
- Create a public const for "pinniped-cli" now that we are using that
  string in several places in the production code

internal/supervisor/server/server.go

@@ -439,6 +439,7 @@ func runSupervisor(ctx context.Context, podInfo *downward.PodInfo, cfg *supervis
 		dynamicUpstreamIDPProvider,
 		&secretCache,
 		clientWithoutLeaderElection.Kubernetes.CoreV1().Secrets(serverInstallationNamespace), // writes to kube storage are allowed for non-leaders
+		clientWithoutLeaderElection.PinnipedSupervisor.ConfigV1alpha1().OIDCClients(serverInstallationNamespace),
 	)
 
 	// Get the "real" name of the client secret supervisor API group (i.e., the API group name with the