have a way to configure publishing dry run checks

[devoncarew]

Right now some packages have intentional dart pub publish --dry-run warnings. firehose fails the build on the dry-run checks; we may want some additional configurability here. See dart-lang/test#1923 for context.

Some ideas are:

• for pub publish dry-run to support some ignore: xxx syntax in pubspec files
• have a way for a repo to configure that dry run checks shouldn't be run for specific packages
• have a way for firehose to distinguish dry-run warnings vs errors and not fail builds for warnings (as pub is currently, any warnings sets an exit code of 65)

[jakemac53]

Fwiw, if the ignore syntax is feasible it would be my preference. This would in general make publishing safer on packages that have known warnings (only new warnings would appear), and document those warnings that are known to exist.

[devoncarew]

One other way we could configure this to ignore publishing warnings is by supporting something like a publishing-ignore-warnings label; that's something a repo maintainer could add to a PR if they knew the particular warnings wasn't relevant.