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
```

store/retrylayer/retrylayer.go

@@ -6121,11 +6121,11 @@ func (s *RetryLayerPostStore) DeleteOrphanedRows(limit int) (int64, error) {
 
 }
 
-func (s *RetryLayerPostStore) Get(ctx context.Context, id string, skipFetchThreads bool, collapsedThreads bool, collapsedThreadsExtended bool, userID string) (*model.PostList, error) {
+func (s *RetryLayerPostStore) Get(ctx context.Context, id string, opts model.GetPostsOptions, userID string) (*model.PostList, error) {
 
 	tries := 0
 	for {
-		result, err := s.PostStore.Get(ctx, id, skipFetchThreads, collapsedThreads, collapsedThreadsExtended, userID)
+		result, err := s.PostStore.Get(ctx, id, opts, userID)
 		if err == nil {
 			return result, nil
 		}