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/post.go

@@ -263,6 +263,9 @@ type GetPostsOptions struct {
 	SkipFetchThreads         bool
 	CollapsedThreads         bool
 	CollapsedThreadsExtended bool
+	FromPost                 string // PostId after which to send the items
+	FromCreateAt             int64  // CreateAt after which to send the items
+	Direction                string // Only accepts up|down. Indicates the order in which to send the items.
 }
 
 func (o *Post) Etag() string {