Validate Scan Policy schema with JSON Schema

What does this MR do?

This change adds additional validation to Scan Policy schema to make sure we are responding with error message when Scan Policy is invalid or missing. The problem was identified during the workflowverification of the related issue: #299215 (comment 555697778) (scenario 6. and 7.)

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Does this MR meet the acceptance criteria?

Conformity

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  • [-] I have included a changelog entry.
  • I have not included a changelog entry because feature behind feature flag.
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Availability and Testing

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• [-] Tested in all supported browsers
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Security

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• [-] 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
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Related to #299215 (closed)