Adds a new command to run dart fix --apply in target packages. Like update-release-info, this is not intended for CI use, but for local development. It's useful when enabling new lint options.

This is the first time a command has been so simple that it is literally just running a single command in all packages without any additional logic. We may at some point want to replace this with a generic "run a provided command string" command instead of having to make commands like this, but since this is the first time it's come up and it's not clear that we'll have other uses for that, this is just adding the specific command and we can re-evaluate if we ever find that we are adding a second command like this.

(We may well also end up adding more complexity here, such as summarizing the number of fixes applied to each package at the end of the output, which is another argument for having a new specific command.)

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I read and followed the relevant style guides and ran the auto-formatter. (Unlike the flutter/flutter repo, the flutter/plugins repo does use dart format.)
• I signed the CLA.
• The title of the PR starts with the name of the plugin surrounded by square brackets, e.g. [shared_preferences]
• I listed at least one issue that this PR fixes in the description above.
• I updated pubspec.yaml with an appropriate new version according to the pub versioning philosophy, or this PR is exempt from version changes.
• I updated CHANGELOG.md to add a description of the change, following repository CHANGELOG style.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or this PR is test-exempt.
• All existing and new tests are passing.