Extract metadata collection to a separate class

What does this MR do?

It refactors issues serializer (which concerts Jira issue to a GitLab issue) and extracts getting metadata part to a separate class.

We want to retrieve also custom fields from the Jira and the serializer could become more complex. This refactoring should avoid it.

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