Set user agent in all requests to connected sources

Currently the User-Agent is set in the requests to the connected sources for some of the source (see for example https://gitlab.com/oersi/oersi-import-scripts/-/blob/master/python/eCampusOntarioImport.py#L60). For some sources, requests are processed without setting a User-Agent.

We should set the User-Agent in all requests to all connected sources.

The User-Agent is configurable in the import-scripts-config and is available in all Import-Classes derived from common.Oersi via self.user_agent

Note: also consider common.SitemapReader and common.OaiPmhReader