imap-send: add ability to list the available folders

imap-send: add ability to list the available folders

Various IMAP servers have different ways to name common folders.
For example, the folder where all deleted messages are stored is often
named "[Gmail]/Trash" on Gmail servers, and "Deleted" on Outlook.
Similarly, the Drafts folder is simply named "Drafts" on Outlook, but
on Gmail it is named "[Gmail]/Drafts".

This commit adds a `--list` command to the `imap-send` tool that lists
the available folders on the IMAP server, allowing users to see
which folders are available and how they are named. A sample output
looks like this when run against a Gmail server:

    Fetching the list of available folders...
    * LIST (\HasNoChildren) "/" "INBOX"
    * LIST (\HasChildren \Noselect) "/" "[Gmail]"
    * LIST (\All \HasNoChildren) "/" "[Gmail]/All Mail"
    * LIST (\Drafts \HasNoChildren) "/" "[Gmail]/Drafts"
    * LIST (\HasNoChildren \Important) "/" "[Gmail]/Important"
    * LIST (\HasNoChildren \Sent) "/" "[Gmail]/Sent Mail"
    * LIST (\HasNoChildren \Junk) "/" "[Gmail]/Spam"
    * LIST (\Flagged \HasNoChildren) "/" "[Gmail]/Starred"
    * LIST (\HasNoChildren \Trash) "/" "[Gmail]/Trash"

For OpenSSL, this is achived by running the 'IMAP LIST' command and
parsing the response. This command is specified in RFC6154:
https://datatracker.ietf.org/doc/html/rfc6154#section-5.1

For libcurl, the example code published in the libcurl documentation
is used to implement this functionality:
https://curl.se/libcurl/c/imap-list.html

Signed-off-by: Aditya Garg <gargaditya08@live.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>