Support for IMAP4rev2 (RFC 9051) / SMTPUTF8 (RFC 6531)

[Otzie2023]

IMAP4rev2 is the latest revision of the IMAP protocol and brings a number of improvements, clarifications and standardisations compared to IMAP4rev1.

Why IMAP4rev2 is important:

1. IMAP4rev2 combines numerous extensions that were previously optional (e.g. ENABLE, UTF8=ACCEPT, LITERAL+, IDLE, etc.). .
2. Many older inconsistencies have been eliminated, simplifying implementations.
3. Modern mail servers (e.g. Dovecot, Cyrus) are already beginning to officially support IMAP4rev2.

Full support would make tokio-imap future-proof and standards-compliant.

Proposal:

1. Gradual implementation of the mandatory extensions
2. Possibly a feature flag imap4rev2 for gradual activation

References:
RFC 9051 – IMAP4rev2
RFC 3501 – IMAP4rev1
RFC 6531 – SMTPUTF8

[djc]

Unless someone wants to fund work on this, I'm not motivated to spend a bunch of time on this. I could probably review some PRs as long as they're easy to review (meaning small commits).

[Otzie2023]

How much would the work cost approximately?

[djc]

How much would the work cost approximately?

Figuring that out is a process by itself that I probably won't be able to do unless funding is available.

[Otzie2023]

Would €30 be enough to implement this?

[djc]

No, it would probably take in the thousands of EUR.

[Alfazalando]

Please provide support for this.