Set version inside application and add to headers

[robbevp]

This PR does two things:

• Adds the application version in an initializer, so it can be used inside the application
• Adds an extra header to the default headers, with the version. (Which will be helpful when debugging in production or if we ever need to handle breaking changes in web/android)

A test didn't really seem relevant, but we can always test that the header is present in a random request