add option to use cancellation token in ChopperClient

[kfiross]

This pull request includes significant changes to the chopper package to add support for request cancellation using cancellation tokens. The changes involve updating various parts of the codebase to integrate the new cancellation_token_http package and modifying existing methods to support cancellation functionality.

Integration of cancellation tokens:

• chopper/lib/src/base.dart: Added a _cancellationToken field and updated the ChopperClient constructor to accept an optional cancellationToken. Modified the execute method to handle cancellation and added a cancelRequests method to cancel ongoing requests. [1] [2] [3] [4] [5] [6]

• chopper/lib/src/chain/call.dart: Updated the execute method to accept an optional cancellationToken parameter and pass it to the HttpCallInterceptor. [1] [2] [3]

Updates to interceptors:

• chopper/lib/src/interceptors/http_call_interceptor.dart: Modified the HttpCallInterceptor to accept an optional cancellationToken and use it when making HTTP calls. [1] [2] [3]

Example and test updates:

• chopper/example/main.dart: Added an example of using the CancellationToken to cancel a request after a delay. [1] [2]
• chopper/test/*: Updated tests to use the cancellation_token_http package instead of the http package. [1] [2] [3] [4] [5] [6] [7]

Dependency updates:

• chopper/pubspec.yaml: Replaced the http dependency with cancellation_token_http.

[techouse]

Hey,

I love the idea of a cancellation token, however, I'm not a fan of using a fork of the http package by default.

I was really hoping for dart-lang/http#978 to be merged by now. 🥺

Could you maybe create an opt-in plugin instead?