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
```

app/plugin_api.go

@@ -661,7 +661,7 @@ func (api *PluginAPI) DeletePost(postID string) *model.AppError {
 }
 
 func (api *PluginAPI) GetPostThread(postID string) (*model.PostList, *model.AppError) {
-	return api.app.GetPostThread(postID, false, false, false, "")
+	return api.app.GetPostThread(postID, model.GetPostsOptions{}, "")
 }
 
 func (api *PluginAPI) GetPost(postID string) (*model.Post, *model.AppError) {