Update Application Settings API to toggle SPP on instance-level

Overview

As we built out Secret Push Protection and took it through an experimental and beta phases, we had a number of gaps in our API support for the feature. We want to address one of those gaps by updating Application Settings API to allow toggling the feature on the instance-level.

Please see this thread for further information.

Proposal

To address this gap, one could possibly:

• Include pre_receive_secret_detection_enabled attribute in EE::ApplicationSettingsHelper's possible_licensed_attributes.
• Update corresponding tests to ensure this works as expected, and is only possible when the feature is licensed.
• Update corresponding API endpoint documentation to indicate this is only possible for Ultimate license.