Extend Maintenance Windows API to allow date+time

Currently creating maintenance window via API only allows for startAt/endAt date while in UI we can specify date and time.

Our use-case is Checkly + our CI/CD: if we want 100% greens, then when we deploy via CI we have to go to Checkly and manually make a maintenance window. It's tedious and easy to forget. Automated maintenance window will solve it, but setting it for the whole day is too much.