fix(ext.bridge): bridge groups failing

[Middledot]

Summary

Fix #1629

Bridge group commands fail because

1. SlashCommandGroups don't have a callback parameter
2. Cogs took the variants of the subcommands of the [bridge] group (e.g. BridgeGroup.subcommands[0].slash_variant) as regular commands which also breaks everything

Information

• This PR fixes an issue.
• This PR adds something new (e.g. new method or parameters).
• This PR is a breaking change (e.g. methods or parameters removed/renamed).
• This PR is not a code change (e.g. documentation, README, typehinting, examples, ...).

Checklist

• I have searched the open pull requests for duplicates.
• If code changes were made then they have been tested.
  • I have updated the documentation to reflect the changes.
• If type: ignore comments were used, a comment is also left explaining why.