[MM-44765] Add API method for getting post's information from permalink (#21518)

* add api for getting post's information

* add information about current user state

Co-authored-by: Mattermod <mattermod@users.noreply.github.com>

api4/post.go

@@ -23,6 +23,7 @@ func (api *API) InitPost() {
 	api.BaseRoutes.Posts.Handle("/ids", api.APISessionRequired(getPostsByIds)).Methods("POST")
 	api.BaseRoutes.Posts.Handle("/ephemeral", api.APISessionRequired(createEphemeralPost)).Methods("POST")
 	api.BaseRoutes.Post.Handle("/thread", api.APISessionRequired(getPostThread)).Methods("GET")
+	api.BaseRoutes.Post.Handle("/info", api.APISessionRequired(getPostInfo)).Methods("GET")
 	api.BaseRoutes.Post.Handle("/files/info", api.APISessionRequired(getFileInfosForPost)).Methods("GET")
 	api.BaseRoutes.PostsForChannel.Handle("", api.APISessionRequired(getPostsForChannel)).Methods("GET")
 	api.BaseRoutes.PostsForUser.Handle("/flagged", api.APISessionRequired(getFlaggedPostsForUser)).Methods("GET")
@@ -1055,3 +1056,24 @@ func getFileInfosForPost(c *Context, w http.ResponseWriter, r *http.Request) {
 	w.Header().Set(model.HeaderEtagServer, model.GetEtagForFileInfos(infos))
 	w.Write(js)
 }
+
+func getPostInfo(c *Context, w http.ResponseWriter, r *http.Request) {
+	c.RequirePostId()
+	if c.Err != nil {
+		return
+	}
+
+	info, appErr := c.App.GetPostInfo(c.AppContext, c.Params.PostId)
+	if appErr != nil {
+		c.Err = appErr
+		return
+	}
+
+	js, err := json.Marshal(info)
+	if err != nil {
+		c.Err = model.NewAppError("getPostInfo", "api.marshal_error", nil, "", http.StatusInternalServerError).Wrap(err)
+		return
+	}
+
+	w.Write(js)
+}