MM-38164: Paginate the GetPostThread API (#19485)

We implement a cursor based pagination model
to page through the posts in a given thread.

The cursor is a combination of the post.CreateAt+
post.Id to differentiate multiple posts in a given
timestamp.

Some additional parameters like direction, fromPost,
fromCreateAt and perPage were introduced to implement
this.

```release-note
NONE
```

services/sharedchannel/permalink_test.go

@@ -27,7 +27,7 @@ func TestProcessPermalinkToRemote(t *testing.T) {
 	utils.TranslationsPreInit()
 
 	pl := &model.PostList{}
-	mockPostStore.On("Get", context.Background(), "postID", true, false, false, "").Return(pl, nil)
+	mockPostStore.On("Get", context.Background(), "postID", model.GetPostsOptions{SkipFetchThreads: true}, "").Return(pl, nil)
 
 	mockStore.On("Post").Return(&mockPostStore)