Add ability to filter result of package GET query by version.

Related to #348690.

According to https://docs.gitlab.com/ee/api/packages.html it's possible to GET a list of packages and filter them by package_name and package_type. However, it's not possible to filter packages by version. This would be very useful (especially while overwriting existing package versions is not possible (#348955)).

So I'd like to request that package_version be added as an available filter on the GET request on the packages API.