Make events cmd work well with lowercased and only kind selector

[somtochiama]

The events cmd didn't return data if the kind in the --for selector was lowercase. This pr fixes that. It also adds validation for the --type and --for flags.

Before

➜  flux2 git:(flux-events) flux events --for alert/webapp    
✗ No events found in flux-system namespace.

Now

 ./bin/flux events --for alert/webapp
LAST SEEN               TYPE    REASON                          OBJECT          MESSAGE                                                                                                                                                 
3h53m                   Warning NotificationDispatchFailed      Alert/webapp    failed to send notification for HelmRepository/flux-system/podinfo: postMessage failed: failed to execute request: context deadline exceeded                   
3h41m                   Warning NotificationDispatchFailed      Alert/webapp    failed to send notification for HelmChart/flux-system/flux-system-flagger: postMessage failed: failed to execute request: context deadline exceeded            
3h2m (x4 over 3h49m)    Warning NotificationDispatchFailed      Alert/webapp    failed to send notification for Kustomization/flux-system/apps: postMessage failed: failed to execute request: context deadline exceeded 

The PR also updates the --for selector now supports specifying only a kind.

 ./bin/flux events --for kustomization --all-namespaces 
NAMESPACE       LAST SEEN       TYPE    REASON                  OBJECT                                  MESSAGE                                                     
flux-system     9h              Normal  ReconciliationSucceeded Kustomization/flux-system               Reconciliation finished in 454.865792ms, next run in 10m0s      
flux-system     9h              Normal  ReconciliationSucceeded Kustomization/flux-system               Reconciliation finished in 519.357167ms, next run in 10m0s      
flux-system     8h              Normal  ReconciliationSucceeded Kustomization/kube-prometheus-stack     Reconciliation finished in 131.935666ms, next run in 1h0m0s     
flux-system     8h              Normal  ReconciliationSucceeded Kustomization/apps                      Reconciliation finished in 77.008625ms, next run in 1h0m0s      

[stefanprodan]

LGTM

Thanks @somtochiama