[WIP] feat: support abortion of in-flight HTTP requests

[JaffaKetchup]

Resolves #424, closes #204, closes #424, closes #567
Closes #978, closes #521, closes #602, closes #819

As discussed in #424, this adds an Abortable interface for BaseRequests that provides an abortTrigger which Clients may use to abort requests.

Currently WIP:

• Compliance tests
• Release prep (changelog etc.)

Complete:

• Basic implementation
• IOClient, BrowserClient, and RetryClient support
• Documentation

We can do it this time :D

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[brianquinlan]

The outline looks good so far! Let me know if you need me to press the button to run the workflows.

[github-actions]

Package publishing

Package	Version	Status	Publish tag (post-merge)
package:cronet_http	1.4.0-wip	WIP (no publish necessary)
package:cupertino_http	2.2.0	already published at pub.dev
package:http	1.4.1-wip	(error) pubspec version (1.4.1-wip) and changelog (1.4.1) don't agree
package:http2	3.0.0	ready to publish	http2-v3.0.0
package:http_multi_server	3.2.2	already published at pub.dev
package:http_parser	4.1.2	already published at pub.dev
package:http_profile	0.1.1-wip	WIP (no publish necessary)
package:ok_http	0.1.1-wip	WIP (no publish necessary)
package:web_socket	1.0.1	already published at pub.dev
package:web_socket_channel	3.0.3	already published at pub.dev

Documentation at https://github.com/dart-lang/ecosystem/wiki/Publishing-automation.

[github-actions]

PR Health

Changelog Entry ❗

Package	Changed Files
package:http	pkgs/http/lib/http.dart pkgs/http/lib/src/abortable.dart pkgs/http/lib/src/base_request.dart pkgs/http/lib/src/browser_client.dart pkgs/http/lib/src/io_client.dart pkgs/http/lib/src/multipart_request.dart pkgs/http/lib/src/request.dart pkgs/http/lib/src/streamed_request.dart

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Coverage ⚠️

File	Coverage
pkgs/http/example/main.dart	💔 Not covered
pkgs/http/lib/http.dart	💚 100 %
pkgs/http/lib/src/abortable.dart	💚 100 %
pkgs/http/lib/src/base_request.dart	💚 88 %
pkgs/http/lib/src/browser_client.dart	💔 Not covered
pkgs/http/lib/src/io_client.dart	💔 83 % ⬇️ 3 %
pkgs/http/lib/src/multipart_request.dart	💔 93 % ⬇️ 4 %
pkgs/http/lib/src/request.dart	💔 96 % ⬇️ 2 %
pkgs/http/lib/src/streamed_request.dart	💔 75 % ⬇️ 25 %

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API leaks ✔️

The following packages contain symbols visible in the public API, but not exported by the library. Export these symbols or remove them from your publicly visible API.

Package	Leaked API symbols

License Headers ⚠️

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// for details. All rights reserved. Use of this source code is governed by a
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Files
pkgs/http/example/main.dart

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