imapserver: implement rfc 9590, returning metadata in the extended list command

only with "return" including "metadata". so clients can quickly get certain
metadata (eg for display, such as a color) for mailboxes.

this also adds a protocol token type "mailboxt" that properly encodes to utf7
if required.

imapclient/parse.go

@@ -460,11 +460,14 @@ func (c *Conn) xuntagged() Untagged {
 			var isString bool
 			if c.take('~') {
 				value = c.xliteral()
-			} else {
+			} else if c.peek('"') {
 				value = []byte(c.xstring())
 				isString = true
 				// note: the abnf also allows nstring, but that only makes sense when the
 				// production rule is used in the setmetadata command. ../rfc/5464:831
+			} else {
+				// For response to extended list.
+				c.xtake("nil")
 			}
 			r.Annotations = append(r.Annotations, Annotation{key, isString, value})
 

imapclient/protocol.go

@@ -37,6 +37,7 @@ const (
 	CapSaveDate         Capability = "SAVEDATE"           // ../rfc/8514
 	CapCreateSpecialUse Capability = "CREATE-SPECIAL-USE" // ../rfc/6154:296
 	CapCompressDeflate  Capability = "COMPRESS=DEFLATE"   // ../rfc/4978:65
+	CapListMetadata     Capability = "LIST-METADTA"       // ../rfc/9590:73
 )
 
 // Status is the tagged final result of a command.
@@ -242,8 +243,10 @@ type UntaggedMetadataKeys struct {
 	Keys []string
 }
 
+// Annotation is a metadata server of mailbox annotation.
 type Annotation struct {
 	Key      string
+	// Nil is represented by IsString false and a nil Value.
 	IsString bool
 	Value    []byte
 }