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

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 Rust-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