Require action to bypass pub publish warnings

[jonasfj]

To bypass a warning in pub publish we should perhaps require a flag like pub publish --by-pass-<name>-warning.

It is very plausible that not all warnings warrants a --by-pass-<name>-warning flag.
Perhaps the authors warning below is a good example of a warning that doesn't require explicit by-passing.

But publishing a package that contains generated dartdoc contents should probably require --by-pass-dartdoc-warning flag. This is almost always harmful, as it takes up a lot of space.

Example

[retry]$pub publish
Publishing retry 3.0.0+1 to https://pub.dartlang.org:
|-- CHANGELOG.md
|-- LICENSE
|-- README.md
|-- analysis_options.yaml
|-- example
|   '-- main.dart
|-- lib
|   '-- retry.dart
|-- mono_pkg.yaml
|-- pubspec.yaml
'-- test
    '-- retry_test.dart
Package validation found the following potential issue:
* Your pubspec.yaml includes an "authors" section which is no longer used
  and may be removed.

Uploads to pub.dev are subject to https://pub.dev/policy

Package has 1 warning.
If you wish to publish anyways by pass the warning with:
  pub publish --by-pass-authors-warning

[jonasfj]

@sigurdm suggested that perhaps the flag should require the warning be present.
So passing --expect-authors-warning will allow publishing with the warning, but forbid publishing if the warning is not present.

This way publishing scripts won't build up a lot of unnecessary --by-pass-X-warning flags, because such flags must be removed once the warning is no-longer present.

[passsy]

That's a fantastic idea and would help me continue running pub publish -n on CI.

Currently, I'm facing the pre-release warning, which is nice, but I explicitly don't want to fail on this because I'm publishing a preview which is completely fine according to the documentation.

I'd love to have an --by-pass-prerelease-warning flag

| Package validation found the following potential issue:
| * Packages with an SDK constraint on a pre-release of the Dart SDK should themselves be published as a pre-release version. If this package needs Dart version 2.12.0-0, consider publishing the package as a pre-release instead.
|   See https://dart.dev/tools/pub/publishing#publishing-prereleases For more information on pre-releases.
| 
| Package has 1 warning.
[Dart CI/lint]   ❌  Failure - Verify package completness
Error: exit with `FAILURE`: 65