Add an API for a scan and a AttrR to only poll once with a ONCE constant

Some attributes need to be updated from the device, but the value will never change. Add a special ONCE constant and update scan and AttrR.update_period to take a float or ONCE. If it is ONCE, add the update callback to initial scan tasks instead of making a task to call it periodically.

## Acceptance Criteria
- Specific criteria that will be used to judge if the issue is fixed
