Use retry callback block for admin API in API exporter tests.

[isoos]

• Investigate why the api exporter test is flaky #8483 - I don't see other reasons for it to be flaky.
• Retry ClientException from package:http #8337 - storage access methods are doing it already, we should continue with pub API uses
• Use package retry for retrying requests #7825 - this method could be a generic callback for such use cases

Note: I would also refactor this method in a follow-up PR (rename to contain the name retry in it, also fn can be regular parameter). Then we should also migrate all tests that are using createPubApiClient and also use it in the application code too