Store SGP30 baseline values as retained MQTT messages.

**Describe the problem you have/What new integration you would like**

The SGP30 is an environmental sensor that measures TVOC and eC02. Once it has been online for 24 hours, the sensor starts outputting baseline values. If these baseline values are provided to the sensor next time it reboots, it will self-calibrate within the hour rather than wait another 24 hours. It would be helpful to have these values automatically stored and retrieved.

**Please describe your use case for this integration and alternatives you've tried:**

The current method is to tail the logs of the node and write down the baseline values and then manually add them to the config file. This can be somewhat tedious to do but, more problematically, if the node unexpectedly loses power (even briefly) or reboots, the readings are useless for the next day.

**Additional context**

In my situation, I am publishing sensor values to a MQTT rather than directly to HA. Publishing the baseline values as a retained message on the MQTT broker is the ideal solution. Either as a separate topic or as a field of the existing config topic. I'm not familiar enough with the native HA API to know how this could be handled using that integration method.

Note: The SGP30 baseline value functionality is broken in the current ESPHome release. It has been corrected on the development branch though. esphome/issues#950