Bitbucket Client and Importer improvements

What does this MR do?

Improved the importer to have the paging cached as descibed in #276443

Furthermore we improved the client and the objects to be more concise and reusable. We introduced three new objects to have a better representation of the Bitbucket Server API.

first separation for !47138 (closed)

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.