Enable TLS 1.3 session resumption for Dovecot

[link2xt]

TLS 1.3 session resumption can be tested by running
openssl s_client -connect c2.testrun.org:443 -tls1_3 -sess_out sess.pem
followed by
openssl s_client -connect c2.testrun.org:443 -tls1_3 -sess_in sess.pem

On the first run you should see New, TLSv1.3, Cipher is TLS_AES_256_GCM_SHA384, but on a second run Reused, TLSv1.3, Cipher is TLS_AES_256_GCM_SHA384. However, second connection also shows "New" even though the server issues two tickets on the first connection as can be seen from two lines Post-Handshake New Session Ticket arrived:.

With Postfix session resumption works but Postfix (port 465) issues only one ticket and only if resumption was not used: #456
Established session can even be reused on port 587 with -starttls smtp and vice versa.
pre_shared_key (41) extension is not encrypted and is visible in Wireshark in the Server Hello message.

nginx sends two tickets if session is not resumed and one ticket if session is resumed.

But with Dovecot on port 993 second run results in New, TLSv1.3, Cipher is TLS_AES_256_GCM_SHA384.

[link2xt]

imap.fastmail.com:993, disroot.org:993, riseup.net:993, imap.migadu.com:993 do not support it, imap.gmail.com:993 supports.