extsvc/bitbucketserver: add function which fetches all repos of a given Bitbucket project
Created by: sashaostrikov
This is used to read all of the repos of a given Bitbucket Project, which is required as a part of https://github.com/sourcegraph/sourcegraph/issues/36355 effort
Closes https://github.com/sourcegraph/sourcegraph/issues/36372
Test plan
New test added