Change BitBucket Pull Request Import to a Batch Process

What does this MR do?

As BitBucket importer tried to fetch all Pull Requests upfront, before importing it, the memory consumption got very high. So high, that bigger projects could never be imported fully, as they had too many Pull Requests.

This change should reduce the amount of occupied memory, as the Pull Requests are now fetched and imported in a batch of 100. As it used to be done before hand, for creating pull request branches.

relates to #23586

Does this MR meet the acceptance criteria?

Conformity

Availability and Testing

https://gitlab.my.salesforce.com/0064M00000XYT8k

Edited by Simon Schrottner