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/post.go

@@ -168,7 +168,7 @@ func (a *App) CreatePost(c *request.Context, post *model.Post, channel *model.Ch
 	if post.RootId != "" {
 		pchan = make(chan store.StoreResult, 1)
 		go func() {
-			r, pErr := a.Srv().Store.Post().Get(sqlstore.WithMaster(context.Background()), post.RootId, false, false, false, "")
+			r, pErr := a.Srv().Store.Post().Get(sqlstore.WithMaster(context.Background()), post.RootId, model.GetPostsOptions{}, "")
 			pchan <- store.StoreResult{Data: r, NErr: pErr}
 			close(pchan)
 		}()
@@ -559,7 +559,7 @@ func (a *App) DeleteEphemeralPost(userID, postID string) {
 func (a *App) UpdatePost(c *request.Context, post *model.Post, safeUpdate bool) (*model.Post, *model.AppError) {
 	post.SanitizeProps()
 
-	postLists, nErr := a.Srv().Store.Post().Get(context.Background(), post.Id, false, false, false, "")
+	postLists, nErr := a.Srv().Store.Post().Get(context.Background(), post.Id, model.GetPostsOptions{}, "")
 	if nErr != nil {
 		var nfErr *store.ErrNotFound
 		var invErr *store.ErrInvalidInput
@@ -841,8 +841,8 @@ func (a *App) GetSinglePost(postID string) (*model.Post, *model.AppError) {
 	return post, nil
 }
 
-func (a *App) GetPostThread(postID string, skipFetchThreads, collapsedThreads, collapsedThreadsExtended bool, userID string) (*model.PostList, *model.AppError) {
-	posts, err := a.Srv().Store.Post().Get(context.Background(), postID, skipFetchThreads, collapsedThreads, collapsedThreadsExtended, userID)
+func (a *App) GetPostThread(postID string, opts model.GetPostsOptions, userID string) (*model.PostList, *model.AppError) {
+	posts, err := a.Srv().Store.Post().Get(context.Background(), postID, opts, userID)
 	if err != nil {
 		var nfErr *store.ErrNotFound
 		var invErr *store.ErrInvalidInput
@@ -887,7 +887,7 @@ func (a *App) GetFlaggedPostsForChannel(userID, channelID string, offset int, li
 }
 
 func (a *App) GetPermalinkPost(c *request.Context, postID string, userID string) (*model.PostList, *model.AppError) {
-	list, nErr := a.Srv().Store.Post().Get(context.Background(), postID, false, false, false, userID)
+	list, nErr := a.Srv().Store.Post().Get(context.Background(), postID, model.GetPostsOptions{}, userID)
 	if nErr != nil {
 		var nfErr *store.ErrNotFound
 		var invErr *store.ErrInvalidInput
@@ -1086,7 +1086,12 @@ func (a *App) GetPostsForChannelAroundLastUnread(channelID, userID string, limit
 		return model.NewPostList(), nil
 	}
 
-	postList, err := a.GetPostThread(lastUnreadPostId, skipFetchThreads, collapsedThreads, collapsedThreadsExtended, userID)
+	opts := model.GetPostsOptions{
+		SkipFetchThreads:         skipFetchThreads,
+		CollapsedThreads:         collapsedThreads,
+		CollapsedThreadsExtended: collapsedThreadsExtended,
+	}
+	postList, err := a.GetPostThread(lastUnreadPostId, opts, userID)
 	if err != nil {
 		return nil, err
 	}
@@ -1533,7 +1538,7 @@ func (a *App) countMentionsFromPost(user *model.User, post *model.Post) (int, in
 	// A mapping of thread root IDs to whether or not a post in that thread mentions the user
 	mentionedByThread := make(map[string]bool)
 
-	thread, err := a.GetPostThread(post.Id, false, false, false, user.Id)
+	thread, err := a.GetPostThread(post.Id, model.GetPostsOptions{}, user.Id)
 	if err != nil {
 		return 0, 0, err
 	}