SwisscomProvider based on GsmaOneApiProvider

Hi Kévin

I did some refactoring...

The abstract ``GsmaOneApiProvider`` contains the parts of the implementation that are described in the gsma specs.  As far as I can tell this part of the code should work with all "gsma oneapi" providers. The ``SwisscomProvider`` configures the right URL and also implements the Swisscom API-Key.

As far as I can tell the unspecified parts of the standards (like auth) are handled different by the various providers.

Even though I'm not 100% sure as to how production ready the Swisscom service is, I believe that this pull request already contains enough substance to be warranted.

Even though this was not done on work time, I should probably disclaim, that I am currently employed by Swisscom.