Allow direct access to the query factory object

Upstream library used by this module, https://github.com/pfrenssen/piwik-reporting-api, has been updated and new options have been added.
Among the new possibilities, is to allow tamper with the Http client's parameters, like the request method.

These new options are offered by the Query factory of the library which is not accessible if one is using the drupal module.

That leaves the side of this module to be updated accordingly. As always, I will propose the minimum viable work needed.

Proposed resolution:
Allow access to the query factory by providing a method in the modules query factory that returns the factory as well. Currently, only a Query object can be retrieved.
This can be done by simply changing the ::getQueryFactory method of the \Drupal\piwik_reporting_api\PiwikQueryFactory from protected, to public, and adding the declaration to the interface too.