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
```

api4/post_test.go

@@ -2184,6 +2184,22 @@ func TestGetPostThread(t *testing.T) {
 	require.Error(t, err)
 	CheckForbiddenStatus(t, resp)
 
+	// Sending some bad params
+	_, resp, err = client.GetPostThreadWithOpts(th.BasicPost.Id, "", model.GetPostsOptions{
+		CollapsedThreads: true,
+		FromPost:         "something",
+		PerPage:          10,
+	})
+	require.Error(t, err)
+	CheckBadRequestStatus(t, resp)
+
+	_, resp, err = client.GetPostThreadWithOpts(th.BasicPost.Id, "", model.GetPostsOptions{
+		CollapsedThreads: true,
+		Direction:        "sideways",
+	})
+	require.Error(t, err)
+	CheckBadRequestStatus(t, resp)
+
 	client.Logout()
 	_, resp, err = client.GetPostThread(model.NewId(), "", false)
 	require.Error(t, err)