Handle and test for different link types in IssueLinksResolver

Related to #234049 (closed)

What does this MR do?

• Added ready? method to Resolvers::Vulnerabilities::IssueLinksResolver to verify if provided link_type is valid, if not then raises an error.
• Added end to end test to ee/spec/requests/api/graphql/vulnerabilities to check if GraphQL is properly returning error when value for link_type is not valid for a vulnerability issue link.

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