WIP: Add JSON schema validations for metrics dashboard

What does this MR do?

In support of #28241 (closed)

This is a second attempt at using JSON schema to validate metrics dashboard schema. See comments in changes for a bit more info on how it works.

Looking for feedback from the team. If we are in agreement I'll go ahead and build out the rest + specs.

@mikolaj_wawrzyniak @rpereira2 (@splattael since you seem to continue to be tagged in these conversations 😄)

Screenshots

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