LibIMAP: Support for the FETCH command (*mostly)

This commit doesn't include support for FETCH BODY, because it's a bit
big already. Rest assured, FETCH is the most complicated IMAP command,
and we'll go back to simple boring ones shortly.

Userland/Libraries/LibIMAP/Parser.h

@@ -60,7 +60,13 @@ private:
 
     ListItem parse_list_item();
 
+    FetchCommand::DataItem parse_fetch_data_item();
+
+    FetchResponseData parse_fetch_response();
+
     StringView parse_literal_string();
+    Optional<Vector<Address>> parse_address_list();
+    Address parse_address();
     StringView parse_astring();
 };
 }