feat: add sqlcmd plugin

[ahyalfan]

Summary

Description:

• Tool repo URL: https://github.com/microsoft/go-sqlcmd
• Plugin repo URL: https://github.com/ahyalfan/asdf-sqlcmd

Checklist

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