List Metadata fields used in messages.
Note
This function lists the message attributes that
ImapCon$fetch_metadata() accepts.
The last three (PREVIEW, SAVEDATE, MODSEQ) are
capability-gated extensions, only sent when the server advertises them.
References
Crispin, M., "Internet Message Access Protocol - Version 4rev1", RFC 2060, doi:10.17487/RFC2060 , December 1996, https://www.rfc-editor.org/info/rfc2060.
