Add an AUTHORS file to each plugin

[stuartmorgan-g]

This ensures that the AUTHORS list is distributed with the plugins whose copyright blocks refer to the project authors, for ease of reference. This is currently a copy of the top-level authors list, but over time they are expected to diverge as people contribute to specific plugins.

This does not bump plugin versions or CHANGLELOGs, since it has no effect on the plugin itself; it will be picked up over time as we ship new versions of plugins.

Part of flutter/flutter#78448

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 Flutter Style Guide and the C++, Objective-C, Java style guides.
• 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.