chore: Ensure that new IMAP extensions can be added in a SemVer compatible way.

[duesee]

Issues

• What happens when we add SORT?
  • Add CommandBody::{Sort,Thread} -> CommandBody is not #[non_exhaustive] -> Breaking change required
  • Add Data::{Sort,Thread} -> Data is not #[non_exhaustive] -> Breaking change required
• What happens when we add \<extension>?
  • ...

Solution

• Use interim features?
  • https://docs.rs/tokio/latest/tokio/#unstable-features
• Make structs #[non_exhaustive]?

[duesee]

Let's go with the interim feature variant 🤞🏻 My gut feeling tells me that this is the most sensible thing to do for IMAP. The alternative would basically mean to make every struct and enum non-exhaustive as I'm not able to overlook how IMAP extensions change the protocol.