Add BaseResponseWithUrl.url

[brianquinlan]

Closes #699
Closes #623
Closes #692
Fixes #293

Adds a new class BaseResponseWithUrl, which can hold a new BaseResponse.url fields until package:http v2 is released.

This is an experiment in package evolution - lets see how it works out!

Users can test whether the new field is present in a BaseResponse. For example:

final client = Client();
final response = client.get(Uri.https('example.com', '/'));
Uri? finalUri;
if (response case BaseResponseWithUrl(:final url)) {
  finalUri = url;
}

// Do something with `finalUri`.
client.close();

Adds BaseResponseWithUrl.url, which is the final URL of the response after following any redirects.

Conformance tests pass in a separate PR:
#1110

As soon as this PR lands, I'll create a follow-up with the "next-breaking-release" label that moves uri to BaseRequest and deprecates BaseResponseWithUrl.

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[brianquinlan]

@AlexV525 Any thoughts about this approach?

[AlexV525]

@AlexV525 Any thoughts about this approach?

The implementation looks simple and good to me. Left some comments above.

[brianquinlan]

FWIW, I renamed BaseResponseV2 to BaseResponseWithUrl to allow for the possibility that we will add more than one new field/method in a minor release and don't want to break users because they add with BaseResponseV2 to their classes!

[brianquinlan]

@lrhn Do you want to provide feedback here? I'd like to land this relatively soon.

[lrhn]

LGTM, but consider using an interface instead of a mixin.