Support non-ascii SMTP user and password strings

Support non-ascii SMTP user and password strings

The user and more importantly the password used when
authenticating SMTP connections MUST be encoded in utf-8, and
'base64-encode-string' requires unibyte strings, so call
'encode-coding-string' on them before base64 encoding them in
case they are multibyte strings.

This applies to the CRAM-MD5, LOGIN, and PLAIN auth methods.
XOAUTH2 access tokens are specified to contain only characters
in the range #x20-#x7E (SPC through ~), so utf-8 encoding is not
necessary.

See RFC 4616 and RFC 4954 (or their later updates).

* lisp/mail/smtpmail.el (smtpmail-try-auth-method): Encode user
and password using utf-8 before base64 encoding.

(Bug#75628)

(cherry picked from commit 745847ba8eca27e981a50ad91b628bbce35bb0f3)