[PLT-7362] Add post' root ID to APIv4 addChannelMember to render added user (as system post) at RHS (#7730)

* add post' root ID to apiv4 addChannelMember to render added user (as system post) at RHS

* add check to post_root_id parameter

* add AddChannelMemberWithRootId function for backward compatibility

model/client4.go

@@ -1663,6 +1663,17 @@ func (c *Client4) AddChannelMember(channelId, userId string) (*ChannelMember, *R
 	}
 }
 
+// AddChannelMemberWithRootId adds user to channel and return a channel member. Post add to channel message has the postRootId.
+func (c *Client4) AddChannelMemberWithRootId(channelId, userId, postRootId string) (*ChannelMember, *Response) {
+	requestBody := map[string]string{"user_id": userId, "post_root_id": postRootId}
+	if r, err := c.DoApiPost(c.GetChannelMembersRoute(channelId)+"", MapToJson(requestBody)); err != nil {
+		return nil, BuildErrorResponse(r, err)
+	} else {
+		defer closeBody(r)
+		return ChannelMemberFromJson(r.Body), BuildResponse(r)
+	}
+}
+
 // RemoveUserFromChannel will delete the channel member object for a user, effectively removing the user from a channel.
 func (c *Client4) RemoveUserFromChannel(channelId, userId string) (bool, *Response) {
 	if r, err := c.DoApiDelete(c.GetChannelMemberRoute(channelId, userId)); err != nil {