[client] add ability to list topic names which exist on the server

[jayshrivastava]

There is no way to list the topics available to be read

Pubsub and kafka libraries (ex. sarama) have this feature

[RobertIndie]

We could list the topics using the pulsaradmin. Here is the API:

pulsar-client-go/pulsaradmin/pkg/admin/topic.go

Lines 43 to 44 in f476814

	// List returns the list of topics under a namespace
	List(utils.NameSpaceName) ([]string, []string, error)

Is that OK for you?

[jayshrivastava]

@RobertIndie I think that's okay. Can you please clarify what to pass into the namespace? When I create a client pulsar.NewClient(...), what namespace does it use by default?

[RobertIndie]

@jayshrivastava The default namespace is public/default.