feat: add asdf sync plugin

[robzr]

Summary

Description: asdf sync is a plugin which implements an asdf extension command to synchronize plugins. It also offers the ability to use Semantic Versioning based version constraints using common constraint syntax, and to specify a plugin URL and/or ref in the comments of .tool-versions.

• Tool repo URL: https://github.com/robzr/asdf-sync
• Plugin repo URL: https://github.com/robzr/asdf-sync

Checklist

• Format with scripts/format.bash
• Test locally with scripts/test_plugin.bash --file plugins/<your_new_plugin_name>
• Your plugin CI tests are green
  • Tip: use the plugin_test action from asdf-actions in your plugin CI