[ext.bridge] Permission decorators and invoke function

[Middledot]

Summary

• A guild_only and has_permissions decorator that can be used for both app and bridge commands for "frontend" and "backend" validation.
• Allow BridgeCommand to be directly invoked (e.g.

@bridge.bridge_command()
async def a_command(self, ctx):
    await other_bridge_cmd.invoke(ctx, ...)

Information

• [ ] This PR fixes an issue.
• [x] 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

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