Added support for Hydreon RG-15 Rain Sensor

[wooti]

Description:

Added support for Hydreon's RG-15 Rain Sensor.

Details of the RS232 communications protocol can be found here.

Checklist:

• The pull request is done against the latest development branch
• Only relevant files were touched
• Only one feature/fix was added per PR and the code change compiles without warnings
• The code change is tested and works with Tasmota core ESP8266 V.2.7.4.9
• The code change is tested and works with Tasmota core ESP32 V.1.0.7.3
• I accept the CLA.

[arendst]

Thx. Pity the sensor is a bit expensive.