Add REST and GraphQL API to retrieve specific issue

What does this MR do?

Add API endpoints (REST & GraphQL) for a single Issue.

This is to support automated anti-abuse systems combating issue spam.

I added both REST and GraphQL endpoints because we're GraphQL-first but our current abuse-mitigation tool accesses the REST API.

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

Locally, you should be able to retrieve an individual issue by these two methods:

1. REST: curl "http://gitlab.dummy:3000/api/v4/issues/1" -H 'PRIVATE-TOKEN: <your API token>'
2. GraphQL:

{
  query: {
     issue(id: "gid://gitlab/Issue/434") {
        title
        description
     }
   }
}

• 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

Closes #37248