cli: Support kubectl-style resource selectors

The various Linkerd subcommands that operate on kubernetes resources all work somewhat differently, and none of them support Kubernetes label selection.

Anywhere where the linkerd CLI operates on a list of resources (tap, top, routes, stat, metrics, etc), linkerd should accept resource selections exactly like kubectl.

Furthermore the --linkerd-namespace flag should be changed so that its short flag is -L, so that the -l flag can be used for label selection (like other kubernetes-related tools).

For example, kubectl supports the following:

:; kubectl get -n linkerd svc/linkerd-web deploy/linkerd-web
NAME                  TYPE        CLUSTER-IP    EXTERNAL-IP   PORT(S)             AGE
service/linkerd-web   ClusterIP   10.0.238.66   <none>        8084/TCP,9994/TCP   11d

NAME                                DESIRED   CURRENT   UP-TO-DATE   AVAILABLE   AGE
deployment.extensions/linkerd-web   1         1         1            1           11d

:; kubectl -n linkerd get po -l linkerd.io/control-plane-component    
NAME                                   READY     STATUS    RESTARTS   AGE
linkerd-controller-58fb7787b-kxhw5     4/4       Running   0          11d
linkerd-grafana-659c48d67-h8lx6        2/2       Running   0          11d
linkerd-identity-84fd9b78b-g9h4c       2/2       Running   0          11d
linkerd-prometheus-644698f444-zg9fn    2/2       Running   0          11d
linkerd-sp-validator-c4965d44c-8qqmb   2/2       Running   0          4d
linkerd-web-7f46b9cfcb-6c6kv           2/2       Running   0          4d

:; kubectl -n linkerd get deploy linkerd-controller linkerd-identity
NAME                 DESIRED   CURRENT   UP-TO-DATE   AVAILABLE   AGE
linkerd-controller   1         1         1            1           11d
NAME                 DESIRED   CURRENT   UP-TO-DATE   AVAILABLE   AGE
linkerd-identity     1         1         1            1           11d

linkerd should accept resource lists similarly.

linkerd stat accepts multiple resources, like kubectl, but does not accept label selectors:

:; linkerd -n linkerd stat deploy linkerd-controller linkerd-identity
NAME                 MESHED   SUCCESS      RPS   LATENCY_P50   LATENCY_P95   LATENCY_P99   TCP_CONN
linkerd-controller      1/1    81.82%   1.1rps           1ms           2ms           3ms         17
linkerd-identity        1/1   100.00%   0.3rps           1ms           1ms           1ms         13

linkerd tap, linkerd top, and likerd routes do not accept a list of resources

:; linkerd -n linkerd tap deploy linkerd-controller linkerd-identity
Error: accepts between 1 and 2 arg(s), received 3
Usage:
  linkerd tap [flags] (RESOURCE)

But there is no technical reason for this, since they all accept all resources of a given type:

:; linkerd -n linkerd tap deploy
req id=0:1 proxy=out src=10.244.3.83:46802 dst=10.244.0.214:4191 tls=no_authority_in_http_request :method=GET :authority=10.244.0.214:4191 :path=/metrics
rsp id=0:1 proxy=out src=10.244.3.83:46802 dst=10.244.0.214:4191 tls=no_authority_in_http_request :status=200 latency=3853µs
end id=0:1 proxy=out src=10.244.3.83:46802 dst=10.244.0.214:4191 tls=no_authority_in_http_request duration=39µs response-length=5635B                                    

The linkerd metrics command does not accept any resource lists, though it supports expanding group types like deploy/foo.

Start by tracing the Linkerd CLI entry points for stat, tap, top, routes, metrics, and the shared namespace flag handling. Compare their resource arguments with kubectl's resource-list and label-selector behavior. Done means the affected commands accept equivalent resource lists and selectors, with --linkerd-namespace using -L, and existing command behavior remains covered.