Make end user explicitly provide user-agent details

[bearloga]

Right now all calls made with the package have the same generic UA pointing at this package.

To make it easier to deal with abuse of WDQS and to comply with https://foundation.wikimedia.org/wiki/Policy:Wikimedia_Foundation_User-Agent_Policy, the user should have to provide an informative, specific UA (e.g. include their name/email, repo URI) of the form: <client name>/<version> (<contact information>)

The package can then append "WikidataQueryServiceR/2.0 (https://github.com/wikimedia/WikidataQueryServiceR/)" to the user-provided UA.

I wan to some contribute in this could you please guide me and elaborate this issue more.