Resolve "Implement gNMI 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 #382

api/proto/gosdn/subscriptionmanagement/subscriptionmanagement.proto

+syntax = "proto3";
+
+package gosdn.subscriptionmanagement;
+
+import "buf/validate/validate.proto";
+import "google/api/annotations.proto";
+import "google/protobuf/descriptor.proto";
+
+option go_package = "code.fbi.h-da.de/danet/gosdn/api/go/gosdn/submanagement";
+
+service SubscriptionManagementService {
+    rpc GetAll(GetAllRequest) returns (GetAllResponse) {
+        option (google.api.http) = {get: "/submanagement/subscriptions"};
+    }
+
+    rpc Get(GetRequest) returns (GetResponse) {
+        option (google.api.http) = {get: "/submanagement/{subid}"};
+    }
+
+    rpc Delete(DeleteRequest) returns (DeleteResponse) {
+        option (google.api.http) = {delete: "/submanagement/{subid}"};
+    }
+
+    rpc Add(AddRequest) returns (AddResponse) {
+        option (google.api.http) = {
+            post: "/submanagement/{mneid}"
+            body: "*"
+        };
+    }
+}
+
+message Subscription{
+    string subid = 1;
+    string pid = 2;
+    string mneid = 3;
+    string mne_name = 4;
+    repeated string paths = 5;
+    SubscribeOptions subscribe_options = 6;
+}
+
+message SubscribeOptions {
+    string gnmi_mode = 1;
+    string gnmi_stream_mode = 2;
+    uint64 sample_interval = 3;
+}
+
+message GetAllRequest{
+    int64 timestamp = 1;
+}
+
+message GetAllResponse{
+    int64 timestamp = 1;
+    repeated Subscription subscriptions = 2;
+}
+
+message GetRequest {
+    int64 timestamp = 1;
+    string subid = 2 [(buf.validate.field).required = true];
+}
+
+message GetResponse {
+    int64 timestamp = 1;
+    Subscription subscriptions = 2;
+}
+
+message DeleteRequest{
+    int64 timestamp = 1;
+    string subid = 2 [(buf.validate.field).required = true];
+}
+
+message DeleteResponse{
+    int64 timestamp = 1;
+}
+
+message AddRequest{
+    int64 timestamp = 1;
+    string mneid = 2 [(buf.validate.field).required = true];
+    Subscription subscription = 3 [(buf.validate.field).required = true];
+}
+
+message AddResponse{
+    int64 timestamp = 1;
+}