Add SORT command support (RFC 5256)

[odrobnik]

Summary

Requesting implementation of the IMAP SORT command (RFC 5256) in swift-nio-imap.

Current state

Some groundwork already exists:

Component	Status
Capability parsing	✅ Capability.context(.sort), Capability.sort(_:), SortKind
Response parsing	✅ SortData model + parseSortData in GrammarParser.swift (line ~2388)
Response encoding	✅ writeSortData in EncodeBuffer
Command encoding	❌ No .sort / .uidSort cases in Command enum
Sort criteria model	❌ No SortKey / SortCriterion type (DATE, FROM, SUBJECT, SIZE, etc.)
Command parser	❌ No parseSort in command parser

So the response side is ready, but there's no way to send a SORT command.

What's needed

1. Sort criteria type — e.g. SortCriterion enum with cases for date, from, subject, to, cc, size, arrival, plus reverse(_:) modifier. RFC 5256 §3:

   sort-key = "ARRIVAL" / "CC" / "DATE" / "FROM" / "REVERSE" SP sort-key / "SIZE" / "SUBJECT" / "TO" / "DISPLAYFROM" / "DISPLAYTO"
   

2. Command cases — .sort and .uidSort on Command:

   case sort(criteria: [SortCriterion], charset: String, key: SearchKey, returnOptions: [SearchReturnOption])
   case uidSort(criteria: [SortCriterion], charset: String, key: SearchKey, returnOptions: [SearchReturnOption])

3. Command encoding — Wire format: SORT (DATE REVERSE SUBJECT) UTF-8 UNSEEN

4. Command parser (for server-side / testing) — Parse the SORT command from wire format.

Motivation

SORT is widely supported (iCloud, Dovecot, many others) and is the most-requested missing IMAP feature in our client library SwiftMail — see Cocoanetics/SwiftMail#11. The response-side support is already in place, so this is primarily about the command side.

References

• RFC 5256 — Internet Message Access Protocol - SORT and THREAD Extensions
• RFC 5267 — Contexts for IMAP4 (ESORT) — extends SORT with return options
• Related capability: SORT=DISPLAY (RFC 5957) for DISPLAYFROM/DISPLAYTO sort keys