Scheduled secret detection scan for security policy

What does this MR do?

Addresses: #334417 (closed)

This MR introduces a new pipeline source security_orchestration_policy which is used to create pipelines from policy defined in Security Policy Project. We want Secret Detection scans to be run according to schedule cadence as mentioned in the policy. Secret Detection scans should run in historic mode for scheduled scan. Historic mode is typically enabled by setting SECRET_DETECTION_HISTORIC_SCAN to true as a CI variable. (See documentation)

How to setup and validate locally (strongly suggested)

Feature documentation: https://docs.gitlab.com/ee/user/application_security/policies/

1. Create a new project with policy file: .gitlab/security-policies/policy.yml

scan_execution_policy:
- name: Scheduled Scan
  description: This policy executes security scan every 10 minutes
  enabled: true
  rules:
  - type: schedule
    cadence: '*/10 * * * *'
    branches: 
      - main
      - master
  actions:
  - scan: secret_detection

1. Enable security_orchestration_policies_configuration for the target project for which the scan policies should be enabled for by doing:

Feature.enable(:security_orchestration_policies_configuration, Project.find(<target_project.id>))

1. Go to Security & Compliance -> Scan Policies for the target project
2. Select Edit policy project and select the policy project that was created in step 1 and click save
3. Wait for one schedule cycle(10-15 mins) and notice that a pipeline with secret_detection job is created

Does this MR meet the acceptance criteria?

Conformity

• I have included changelog trailers, or none are needed. (Does this MR need a changelog?)
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• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.
• This change is backwards compatible across updates, or this does not apply.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
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Security

Does this MR contain changes to processing or storing of credentials or tokens, authorization and authentication methods or other items described in the security review guidelines? If not, then delete this Security section.

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team