allow a custom connectivity check

[josh-burton]

Our app uses Dio along with the native adapter, so it uses cupertino http and cronet under the hood.

This means the connectivity checks are using a different stack to our standard http requests.

It would be great to be able to configure internet_connectivity_plus to use the same setup. I think a http Client parameter would work, or the option to support a custom _checkReachabilityFor method.