Return confidence from OpenCage API

[Brammm]

We have a business case where we would need the accuracy/confidence from the providers we are using. This PR adds this for OpenCage.

[freyfogle]

HI @Brammm Ed from OpenCage here. Thanks for this change, I am all for it. But one thing for your project, because unfortunately many people get confused by our confidence value - I just want to make sure you understand it is not a measure of "correctness". It is "just" a measure of how large the result is. Please see the details in the API docs.

[Brammm]

Hey @freyfogle, thanks for that heads up, I really appreciate it. We're converting the OpenCage confidence to an generalized "accuracy" in our app, but for our use case, Confidence as defined does work (e.g. with Google Maps, we're using the location type).