[kn-admin] As a Knative administrator, I want to enable namespace flag on registry operations

[12345lcr]

Now we have kn admin registry add and kn admin registry rm to manage registry credentials on default namespace. I want to add --namespace or -n flag to kn admin registry so that registry credentials on other namespace can also be added and removed. And also add --serviceaccount or -s flag to kn admin registry to enable choosing service account.

For example:
Manage registry used by Knative service deployment

Usage:
  kn admin registry [command]

Available Commands:
  add         Add registry with credentials
  help        Help about any command
  remove      Remove registry settings

Flags:
  -h, --help                    help for registry
  -n, --namespace string        the namespace to manage registries
  -s, --serviceaccount string   the serviceaccount to save imagePullSecrets

Global Flags:
      --config string   config file (default is $HOME/.config/kn/plugins/admin.yaml)

Use "kn admin registry [command] --help" for more information about a command.

[12345lcr]

/assign I would work on this new flag feature.