Add delete cmd for Pulsar Functions (streamnative/pulsar-admin-go#26)

Signed-off-by: xiaolong.ran <[email protected]>

Master Issue: streamnative/pulsar-admin-go#2 

Add delete cmd for Pulsar Functions, the output as follows:

```
USED FOR:
    This command is used for delete a Pulsar Function that is running on a Pulsar cluster.

REQUIRED PERMISSION:
    This command requires super-user permissions.

EXAMPLES:
    #Delete a Pulsar Function that is running on a Pulsar cluster
    pulsarctl functions create
	--tenant public
	--namespace default
	--name <the name of Pulsar Functions>

    #Delete a Pulsar Function that is running on a Pulsar cluster with instance ID
    pulsarctl functions create
	--tenant public
	--namespace default
	--name <the name of Pulsar Functions>
	--instance-id 1

    #Delete a Pulsar Function that is running on a Pulsar cluster with FQFN
    pulsarctl functions delete
	--fqfn tenant/namespace/name [eg: public/default/ExampleFunctions]

OUTPUT:
    #normal output
    Deleted successfully

Usage: pulsarctl functions delete [flags]

Aliases: delete, delete

FunctionsConfig flags:
      --fqfn string        The Fully Qualified Function Name (FQFN) for the function
      --tenant string      The tenant of a Pulsar Function
      --namespace string   The namespace of a Pulsar Function
      --name string        The name of a Pulsar Function
```

pulsaradmin/pkg/pulsar/functions.go

@@ -49,6 +49,8 @@ type Functions interface {
 
 	// Stop function instance
 	StopFunctionWithID(tenant, namespace, name string, instanceID int) error
+
+	DeleteFunction(tenant, namespace, name string) error
 }
 
 type functions struct {
@@ -183,21 +185,17 @@ func (f *functions) CreateFuncWithUrl(funcConf *FunctionConfig, pkgUrl string) e
 
 func (f *functions) StopFunction(tenant, namespace, name string) error {
     endpoint := f.client.endpoint(f.basePath, tenant, namespace, name)
-    err := f.client.post(endpoint+"/stop", "", nil)
-    if err != nil {
-        return err
-    }
-    return nil
+    return f.client.post(endpoint+"/stop", "", nil)
 }
 
 func (f *functions) StopFunctionWithID(tenant, namespace, name string, instanceID int) error  {
     id := fmt.Sprintf("%d", instanceID)
     endpoint := f.client.endpoint(f.basePath, tenant, namespace, name, id)
 
-    err := f.client.post(endpoint+"/stop", "", nil)
-    if err != nil {
-        return err
-    }
+    return f.client.post(endpoint+"/stop", "", nil)
+}
 
-    return nil
+func (f *functions) DeleteFunction(tenant, namespace, name string) error {
+	endpoint := f.client.endpoint(f.basePath, tenant, namespace, name)
+	return f.client.delete(endpoint, nil)
 }