Allow the dependency validation feature flag to be selectively disabled

What does this MR do?

This MR allows the dependency validation feature flag to be selectively disabled.

• ci_validate_build_dependencies is the main switch for the feature, which is enabled by default.
• ci_validate_build_dependencies_override is the switch to disable the feature on a particular project.

The current status of this feature is:

• The feature is enabled already on on-premises instances.
• The feature is disabled globally on gitlab.com.

The end-goal is to enable this on gitlab.com and eventually remove the feature flags. We need to gradually rollout the feature with the _override safe guard to revert the behavior on specific projects if user complains.

Related #257852 (closed)

TODO

• Enable ci_validate_build_dependencies_override feature flag on gitlab.com before merging this in order to keep the current feature state. => https://gitlab.slack.com/archives/C101F3796/p1610951278119600

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• 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