[Core] Allow dropping and elevating permissions

# Feature request

#### Select the type of feature you are requesting:

- [ ] Cog
- [x] Command
- [ ] API functionality
- [x] Core bot functionality

#### Describe your requested feature

`[p]sudo`/`[p]unsudo` commands (actual names might be different when this gets added) that would allow owners to elevate their permissions to be able to use owner-only commands and drop their permissions to be treated as if they weren't bot owner. Functionality would be optional.

From original issue (#3457):
> Now that teams are here, people using their features may want to be able to have the bot treat them like a normal user most of the time but elevate to owner permissions if they are allowed to do so as needed.

> This one may need more discussion before any implementation as it would likely need to be an always available command with inlined checks to prevent lockouts.