Add Jira issues show mock endpoint and add basic frontend structure

What does this MR do?

These changes are early work behind the feature flag jira_issues_show_integration.

• Add backend endpoint with mock data that the frontend expects.
• Add frontend logic to fetch from the above endpoint.
• Add basic frontend components to allow splitting of the rest of the work into smaller components.

Progress

MR	header
Render empty Jira issue in GitLab (behind feature flag)	✅ Done
Add Jira issues show mock endpoint and add basic frontend structure	👈 You are here
Add sidebar to individual Jira Issue show	🚧 WIP
Add custom breadcrumb for Jira issues show	🚧 WIP
Add GlAlert with link to Jira issue	🚧 WIP
More MRs to come	...

How to test this

• Enable the Jira integration and make sure you check "Enable Jira issues".
• Enable the jira_issues_show_integration feature flag.
• Go to Project > Jira Issues > Issue list, add any id to the URL so it looks something like this http://127.0.0.1:3000/gitlab-org/gitlab-shell/-/integrations/jira/issues/1

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