Environment variable for API versioning in Shopware

[leichteckig]

In #18 the need for an environment variable for Shopware's API versions came up, see quote:

Personally I'm not a big fan of changing the API version this many times. We can't ensure the project which uses this package supports the hardcoded API version. As a proposal to solve the problem we should come up with an environment / config variable which contains the API version the test platform should use.

This way the project which uses this package can define the version they would like to use instead of constantly bumping up the API version in the URLs itself.

See:

* https://docs.cypress.io/guides/guides/environment-variables.html#Setting