[local_auth] Move local_auth plugin into a subfolder in preparation for federation migration

[BeMacized]

This PR is a precursor to upcoming PRs for migrating the local_auth plugin to a federated architecture.

This PR only moves the local_auth plugin to a subfolder to make room for the native and platform interface packages.

Relevant issues:

• Federate the Android and iOS implementations of 1P plugins flutter#68498

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].
  • Not needed, is not a new release. No code has been changed.
• I updated CHANGELOG.md to add a description of the change, [following repository CHANGELOG style].
  • Not needed, is not a new release. No code has been changed.
• I updated/added relevant documentation (doc comments with ///).
  • Not needed, is not a new release. No code has been changed.
• I added new tests to check the change I am making, or this PR is [test-exempt].
  • Not needed, is not a new release. No code has been changed.
• All existing and new tests are passing.

[mvanbeusekom]

LGTM