Allow specifying that SMTP auth should always be used

* doc/misc/smtpmail.texi (Authentication): Document it.

* lisp/mail/smtpmail.el
(smtpmail-servers-requiring-authorization): New variable (bug#26359).
(smtpmail-via-smtp): Use it.
1 job for master in 90 minutes and 2 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Test
failed #2711
test-all

01:30:02

 
Name Stage Failure
failed
test-all Test The script exceeded the maximum execution time set for the job
  GEN      lisp/auth-source-pass-tests.log
ELC lisp/auth-source-tests.elc
GEN lisp/auth-source-tests.log
ELC lisp/autoinsert-tests.elc
GEN lisp/autoinsert-tests.log
ELC lisp/autorevert-tests.elc
GEN lisp/autorevert-tests.log
Pulling docker image gitlab/gitlab-runner-helper:x86_64-d0b76032 ...
ERROR: Job failed: execution took longer than 1h30m0s seconds