[BE] WebIDE: Custom schema validation [RUN AS-IF-FOSS]

What does this MR do?

Issue: #226982 (closed)

Note: This MR includes the backend logic only. Frontend logic is added in !41700 (merged)

Add support for validating custom JSON and YAML files in the Web IDE using schemas defined in the .gitlab/.gitlab-webide.yml file.

You can test it out on any project by adding the following to the above file:

schemas:
  - uri: https://json.schemastore.org/package
    match: 
      - package.json

This is an Enterprise Edition feature available only for GitLab Premium and above. So you would need an appropriate license to test this out.

Note: This MR also fixes issues #232761 (closed).

The schemas not loading in Google Chrome is the prime reason we implemented this backend proxy approach. Additionally this logic in the backend does not need YAML parsing library to be loaded in the frontend thus saving a couple of bytes and processing in the Web IDE frontend.

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