Extract Gitlab::Ci::Lint class

What does this MR do?

This MR introduces a Gitlab::Ci::Lint class that represents the SSOT of the CI Lint feature. It provides an abstraction layer that encapsulates various strategies that the CI Lint can run against: static validation or pipeline simulation (a.k.a dry-run).

This class returns a result object that is agnostic from which mode was used and makes it easy to build serializer for it or to display it in the UI. E.g. for #231352 (closed) where both modes are supported.

This is related to !37344 (comment 395471537)

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