Resolve "Implement missing functionality of subscription management"

Description

Related Issue

Motivation and Context

How Has This Been Tested?

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

Related to #385

controller/api/subManagement.go

@@ -35,3 +35,49 @@ func GetAll(ctx context.Context, addr string) (*subpb.GetAllResponse, error) {
 
 	return subClient.GetAll(ctx, req)
 }
+
+// Get fetches detailed info for one specific available subscription.
+func Get(ctx context.Context, addr string, subID string) (*subpb.GetResponse, error) {
+	subClient, err := nbi.SubManagementClient(addr, dialOptions...)
+	if err != nil {
+		return nil, err
+	}
+
+	req := &subpb.GetRequest{
+		Timestamp: time.Now().UnixNano(),
+		Subid:     subID,
+	}
+
+	return subClient.Get(ctx, req)
+}
+
+// Delete stops and removes one specific running subscription is a no-op if object connected to ID does not exist.
+func Delete(ctx context.Context, addr string, subID string) (*subpb.DeleteResponse, error) {
+	subClient, err := nbi.SubManagementClient(addr, dialOptions...)
+	if err != nil {
+		return nil, err
+	}
+
+	req := &subpb.DeleteRequest{
+		Timestamp: time.Now().UnixNano(),
+		Subid:     subID,
+	}
+
+	return subClient.Delete(ctx, req)
+}
+
+// Add creates a new subscription for the network element matching the provided ID using the provided subscribe options.
+func Add(ctx context.Context, addr string, mneID string, subscribeOptions *subpb.Subscription) (*subpb.AddResponse, error) {
+	subClient, err := nbi.SubManagementClient(addr, dialOptions...)
+	if err != nil {
+		return nil, err
+	}
+
+	req := &subpb.AddRequest{
+		Timestamp:    time.Now().UnixNano(),
+		Mneid:        mneID,
+		Subscription: subscribeOptions,
+	}
+
+	return subClient.Add(ctx, req)
+}