Add microseconds as a unit of duration

[victorclaessen]

I opened PR #140307 and then was referred to the architecture discussion section.

I would like to add microseconds as a unit to the duration device_class.

As @jbouwh pointed out, it is already in the converter because (if I understand correctly) it used to be an existing time constant before the duration device_class was implemented (discussion #630).

[jbouwh]

I assume you want to add microseconds (µs) a s valid unit for SensorDeviceClass.DURATION

[abmantis]

We have discussed this with the core team and agreed that it is OK to add the unit to NumberDeviceClass.DURATION and SensorDeviceClass.DURATION.
Thanks!