[Improve] Add admin api GetListActiveBrokers (#1212)

### Motivation

To keep consistent with the [Java client](https://github.com/apache/pulsar/pull/14702).


### Modifications

Add admin api GetListActiveBrokers
diff --git a/pulsaradmin/pkg/admin/brokers.go b/pulsaradmin/pkg/admin/brokers.go
index 650fab8..7dcea80 100644
--- a/pulsaradmin/pkg/admin/brokers.go
+++ b/pulsaradmin/pkg/admin/brokers.go
@@ -27,6 +27,9 @@
 
 // Brokers is admin interface for brokers management
 type Brokers interface {
+
+	// GetListActiveBrokers Get the list of active brokers in the local cluster.
+	GetListActiveBrokers() ([]string, error)
 	// GetActiveBrokers returns the list of active brokers in the cluster.
 	GetActiveBrokers(cluster string) ([]string, error)
 
@@ -86,6 +89,16 @@
 	return res, nil
 }
 
+func (b *broker) GetListActiveBrokers() ([]string, error) {
+	endpoint := b.pulsar.endpoint(b.basePath)
+	var res []string
+	err := b.pulsar.Client.Get(endpoint, &res)
+	if err != nil {
+		return nil, err
+	}
+	return res, nil
+}
+
 func (b *broker) GetDynamicConfigurationNames() ([]string, error) {
 	endpoint := b.pulsar.endpoint(b.basePath, "/configuration/")
 	var res []string
diff --git a/pulsaradmin/pkg/admin/brokers_test.go b/pulsaradmin/pkg/admin/brokers_test.go
index 9767975..3ae9e4a 100644
--- a/pulsaradmin/pkg/admin/brokers_test.go
+++ b/pulsaradmin/pkg/admin/brokers_test.go
@@ -58,3 +58,18 @@
 	assert.NotEmpty(t, leaderBroker.ServiceURL)
 	assert.NotEmpty(t, leaderBroker.BrokerID)
 }
+
+func TestGetAllActiveBrokers(t *testing.T) {
+	readFile, err := os.ReadFile("../../../integration-tests/tokens/admin-token")
+	assert.NoError(t, err)
+	cfg := &config.Config{
+		Token: string(readFile),
+	}
+	admin, err := New(cfg)
+	assert.NoError(t, err)
+	assert.NotNil(t, admin)
+
+	brokers, err := admin.Brokers().GetListActiveBrokers()
+	assert.NoError(t, err)
+	assert.NotEmpty(t, brokers)
+}