Normalized schema: add support for rain/precipitations

[colinsurprenant]

Related to #395

Summary

The SenseCAP 2120 has a rain gauge & rain sensor which provides data for rainfall Intensity in mm/hour and cumulative rainfall in mm.

Why do we need this?

Precipitations/rain data in not part of the normalized schema but rainfall data is common in environmental monitoring and in weather monitoring all-in-one devices like the SenseCAP 2120.

A vendor provided decoder also exists for the S2120 v2 firmware which support these fields.

What is missing? What do you want to see?

I suggest adding a rain or rainfall object with the rainfallIntensity and cumulativeRainfall fields.

How do you propose to implement this?

I suppose speed in m/s could be used for rainfallIntensity which is in mm/s but it doesn't feel quite right- this is not rain speed per se but more a concept of accumulation speed. Maybe introduce a new quantity that better fit this?
Same idea with depth in cm that could be used for cumulativeRainfall in mm.

Can you do this yourself and submit a Pull Request?

Yes if/when there is support for this proposal.