[flutter_plugin_tools] Add a new base command for looping over packages

[stuartmorgan-g]

Most of our commands are generally of the form:

  for (each plugin as defined by the tool flags)
    check some things for success or failure
  print a summary all of the failing things
  exit non-zero if anything failed

Currently all that logic not consistent, having been at various points copied and pasted around, modified, in some cases rewritten. There's unnecessary boilerplate in each new command, and there's unnecessary variation that makes it harder both to maintain the tool, and to consume the test output:

• There's no standard format for separating each plugin's run to search within a log
• There's no standard format for the summary at the end
• In some cases commands have been written to ToolExit on failure, which means we don't actually get the rest of the runs

This makes a new base class for commands that follow this structure to use, with shared code for all the common bits. This makes it harder to accidentally write new commands incorrectly, easier to maintain the code, and lets us standardize output so that searching within large logs will be easier.

This ports two commands over as a proof of concept to demonstrate that it works; more will be converted in follow-ups.

Related to flutter/flutter#83413

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. (Note that 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.
• I updated CHANGELOG.md to add a description of the change.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making or feature I am adding, or Hixie said the PR is test exempt.
• All existing and new tests are passing.

[gaaclarke]

LGTM, just a handful of nits