Add attribute "last triggered alarm" to alarmo entity

[Gifford47]

Checklist

• I checked for similar existing requests (both open and closed) before posting.
• My request is generic, other users may benefit from it too.

Proposal

I want to derive the last triggered alarm time by a template. It would be very easy to get this time (and date) from an attribute of the alarmo entity which is refreshed everytime the alarm triggers.

Additional info

It's related to #642 and #960

Please add an extra attribute with the last triggered alarm time as datetime type.