Make workday sensor configurable due to lack of calender option in systemsensor

[ImC0mmanda]

Checklist

• I checked for similar existing requests (both open and closed) before posting.
• My request is generic, other users may benefit from it too.
• I will participate in further discussion about this feature and can test it (if requested) once it's done.

Proposal

It is not possible to configure what workday sensor (entity) is used.
The system sensor does not consider calendars (unbelievable - but another topic) so most people would use a templatesensor for a proper workday sensor.

Additional info

Please make the entity of the used workdaysensor configurable.
There are several options configurable through yaml, that would be finde for the used workdaysensor-entity too.