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
```

model/client4.go

@@ -3748,6 +3748,49 @@ func (c *Client4) GetPostThread(postId string, etag string, collapsedThreads boo
 	return &list, BuildResponse(r), nil
 }
 
+// GetPostThreadWithOpts gets a post with all the other posts in the same thread.
+func (c *Client4) GetPostThreadWithOpts(postID string, etag string, opts GetPostsOptions) (*PostList, *Response, error) {
+	urlVal := c.postRoute(postID) + "/thread"
+
+	values := url.Values{}
+	if opts.CollapsedThreads {
+		values.Set("collapsedThreads", "true")
+	}
+	if opts.CollapsedThreadsExtended {
+		values.Set("collapsedThreadsExtended", "true")
+	}
+	if opts.SkipFetchThreads {
+		values.Set("skipFetchThreads", "true")
+	}
+	if opts.PerPage != 0 {
+		values.Set("perPage", strconv.Itoa(opts.PerPage))
+	}
+	if opts.FromPost != "" {
+		values.Set("fromPost", opts.FromPost)
+	}
+	if opts.FromCreateAt != 0 {
+		values.Set("fromCreateAt", strconv.FormatInt(opts.FromCreateAt, 10))
+	}
+	if opts.Direction != "" {
+		values.Set("direction", opts.Direction)
+	}
+	urlVal += "?" + values.Encode()
+
+	r, err := c.DoAPIGet(urlVal, etag)
+	if err != nil {
+		return nil, BuildResponse(r), err
+	}
+	defer closeBody(r)
+	var list PostList
+	if r.StatusCode == http.StatusNotModified {
+		return &list, BuildResponse(r), nil
+	}
+	if jsonErr := json.NewDecoder(r.Body).Decode(&list); jsonErr != nil {
+		return nil, nil, NewAppError("GetPostThread", "api.unmarshal_error", nil, jsonErr.Error(), http.StatusInternalServerError)
+	}
+	return &list, BuildResponse(r), nil
+}
+
 // GetPostsForChannel gets a page of posts with an array for ordering for a channel.
 func (c *Client4) GetPostsForChannel(channelId string, page, perPage int, etag string, collapsedThreads bool) (*PostList, *Response, error) {
 	query := fmt.Sprintf("?page=%v&per_page=%v", page, perPage)