Allow variables in `offset` of `sun`-based triggers

It would be nice to be able to have a dynamic offset when using the sun platform as a trigger. With the advent of blueprints, this becomes essential in some scenarios.

Right now this:

  - platform: sun
    event: sunrise
    offset: "{{ some_logic_here }}"

Does not work.

Use the sun elevation instead. That gives you a consistent light level through the year - -3 degrees is the same light level on the shortest day as the longest day.

That’s a good suggestion consistency-wise. But it’s hard to relay the significance of -x or x to blueprint users. I know this may sound condensing, but I myself don’t know how my automation will work if I use -3.

Regardless of the good suggestion, I think offset should accept variables.