Label Jira imported issues with an auto-generated label

What does this MR do?

Generate a label to mark newly imported issues, so that it is easier for the team to distinguish between imported issues and the ones already created in gitlab.

Related issue #210012 (closed) Jira Import Epic for context: &2766 (closed)

NOTE:

• The whole jira import process is behind feature flag, to help us try it out and demo prior releasing it to GA

Screenshots

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