Missing SWIG bindings for repo::DownloadCallbacks "user_data"

[jrohel]

This issue is a follow-up to #1441, which was resolved in PR #1849.

The problem is that in addition to user_cb_data, there is another argument of type void * - user_data. It is passed to the target language as input in the repo::DownloadCallbacks::add_new_download method. Its value can be set for different downloaders:

repo::FileDownloader: user_data can be passed as a parameter to the add method for the file being added.
repo::PackageDownloader: user_data can be passed as a parameter to the add method for the package being added.

Methods repo::Repo::set_user_data and repo::Repo::get_user_data to set/get user_data for each repository - used when downloading repository metadata.

Support for user_data is completely missing in SWIG bundings.