Improve response from Jira issue creation from vulnerability

What does this MR do?

Related to #280951 (closed) and https://sentry.gitlab.net/gitlab/gitlabcom/issues/2418495/?query=is%3Aunresolved

This change fixes problem with Internal Server Error from GraphQL mutation when creating external issue link. To avoid additionally calling 3rd party API to fetch information about Jira issue, we will return now empty response for external issue and fetch it in background using https://gitlab.com/gitlab-org/gitlab/blob/334beb400433c0ac8986cfe68c7b8400a126520e/ee%2Fapp%2Ffinders%2Fprojects%2Fintegrations%2Fjira%2Fby_ids_finder.rb#L6 and ReactiveCaching.

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry => no need, fix for the issue introduced in this release + feature hidden behind feature flag.
• 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