contrib: add pull_logger extension

This extension logs the pull parameters, i.e. the remote and common heads,
when pulling from the local repository.

The collected data should give an idea of the state of a pair of repositories
and allow replaying past synchronisations between them. This is particularly
useful for working on data exchange, bundling and caching-related
11 jobs for topic/default/pull_logger in 73 minutes and 8 seconds (queued for 4 seconds)