Add more maintenance options to safer-cluster module

[christiangonre]

TL;DR

The beta private cluster module used from safer-cluster module has an option to enable maintenance_end_time, maintenance_exclusions and maintenance_recurrence.
It would be awesome if we could specify it from the safer-cluster module too.

Terraform Resources

No response

Detailed design

I will copy those variables from the module beta-private-cluster and enable it on the safer-cluster module

Additional information

I will commit a pull request with the solution. Thanks for these modules, they are fantastic!