[Proposal] Improve the JSON with a `timestamp` type

[nielsbasjes]

In several models there are points that represent an instant (a moment in time) and which are always a 2 register uint32 which describe that instant as the number of seconds since 2000-01-01T00:00:00Z

From a code generation standpoint it is hard to see which of the fields are this.

Proposal: In the Json representation add a new value for type called timestamp which simply means that the 2 registers must be parsed as a uint32 and have the meaning of the number of seconds since 2000-01-01T00:00:00Z.

This proposal does not change anything the actual specification or what the devices are providing.
This is just an improvement in the way the specification is documented which will make using the specification easier.

[nielsbasjes]

My SMA inverter only have model 122 which has a timestamp: Simply a uint32 which is the seconds since 2000.

I just noticed that some models (I found 3,4,5,6,7,9 and 220) which have a uint32 "timestamp in seconds"; it is immediately followed by a milliseconds point.
Does that mean these two combined represent the timestamp in ms ?