Create and register a new push check

Overview

To scan secrets on every push, we have to introduce a new push check. It should be invoked in EE::Gitlab::Checks::PushRuleCheck.

It is also necessary to have the check behind a feature flag, so we don't cause any disruption to our "hot path".

Please see parent epic for more details and considerations.

Implementation Plan

• Create a new push check class, e.g. SecretsCheck, see POC for reference.
• Update EE::Gitlab::Checks::PushRuleCheck to run the new check sequentially and in parallel.
• Create a new feature flag, see documentation for guidelines.
• Ensure the new push check is behind the feature flag.

Based on outcome of #427041 (closed):

• Ensure the push check is also behind the instance-level configuration.

Notes

• The push check doesn't have to do anything at this moment, it can be empty.