Added last weight sensor.

[dckiller51]

@edenhaus
Hello Robert, After turning the subject over and over, to get the last date and time, I decided to add the last weigh sensor as an external addition. This can be added in the configuration. Then it creates a sensor and adds the attribute. This seems to me the simplest and most reliable. I added examples to help the community create persistent sensors, one of the user errors to have this component functional all the time. What do you think? I like the idea of ​​having all the data grouped in a single device even if we duplicate the sensors.

My files are ready to be published on the main branch. I'll try to make a pull request instead.