Merge oauth2 package

[mosuem]

Merge package:oauth2 into this repo.

---

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[github-actions]

PR Health

Changelog Entry ✔️

Package	Changed Files

Changes to files need to be accounted for in their respective changelogs.

Coverage ⚠️

File	Coverage
pkgs/oauth2/example/main.dart	💔 Not covered
pkgs/oauth2/lib/oauth2.dart	💔 Not covered
pkgs/oauth2/lib/src/authorization_code_grant.dart	💚 91 %
pkgs/oauth2/lib/src/authorization_exception.dart	💚 13 %
pkgs/oauth2/lib/src/client.dart	💚 87 %
pkgs/oauth2/lib/src/client_credentials_grant.dart	💚 93 %
pkgs/oauth2/lib/src/credentials.dart	💚 94 %
pkgs/oauth2/lib/src/expiration_exception.dart	💚 50 %
pkgs/oauth2/lib/src/handle_access_token_response.dart	💚 90 %
pkgs/oauth2/lib/src/parameters.dart	💚 88 %
pkgs/oauth2/lib/src/resource_owner_password_grant.dart	💚 93 %
pkgs/oauth2/lib/src/utils.dart	💚 100 %

This check for test coverage is informational (issues shown here will not fail the PR).

This check can be disabled by tagging the PR with skip-coverage-check.

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 ✔️

// Copyright (c) 2024, the Dart project authors. Please see the AUTHORS file
// for details. All rights reserved. Use of this source code is governed by a
// BSD-style license that can be found in the LICENSE file.

Files
no missing headers

All source files should start with a license header.

Package publish validation ✔️

Package	Version	Status
package:oauth2	2.0.3	ready to publish
package:cli_config	0.2.1-wip	WIP (no publish necessary)
package:extension_discovery	2.0.1-wip	WIP (no publish necessary)
package:graphs	2.3.3-wip	WIP (no publish necessary)
package:source_map_stack_trace	2.1.2	already published at pub.dev
package:unified_analytics	6.1.3-wip	WIP (no publish necessary)

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

[github-actions]

Package publishing

Package	Version	Status	Publish tag (post-merge)
package:oauth2	2.0.3	ready to publish	oauth2-v2.0.3
package:cli_config	0.2.1-wip	WIP (no publish necessary)
package:extension_discovery	2.0.1-wip	WIP (no publish necessary)
package:graphs	2.3.3-wip	WIP (no publish necessary)
package:source_map_stack_trace	2.1.2	already published at pub.dev
package:unified_analytics	6.1.3-wip	WIP (no publish necessary)

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