Define CI static validation extension for Editor Lite

What does this MR do?

Split from !49237 (closed).

This MR adds a new validation extension for editor lite, to be used via the .use() API of the editor.

Extension usage

Given the project root/project1 and the ref master (can be any commit sha). On the editor instance, the extension can be used by running following:

import { CiSchemaExtension } from '~/editor/editor_ci_schema_ext';

// ...

editorInstance.use(new CiSchemaExtension());
editorInstance.registerCiSchema({
  projectPath: 'project1',
  projectNamespace: 'root',
  ref: 'master',
});

After which, the validation schema is loaded in the background and the syntax validation can be used.

No changelog is needed, as this is a background improvement required for #263145 (closed)

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

Related to #263145 (closed)