ReceivedHeader: add parse for Authenticated sender

[toto4ds]

Hi,

Looking at ticket #78, I propose to add parsing "Authenticated sender", example:

Received: from my.mail.host (my.mail.host [193.193.193.193])
(using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits)
key-exchange X25519 server-signature RSA-PSS (2048 bits) server-digest SHA256)
(No client certificate requested)
(Authenticated sender: postmaster@my.mail.host)
by my.mail.host (Postfix) with ESMTPSA id 1F2D84BE6E0E
for postmaster@my.mail.host; Tue, 14 Jan 2020 15:48:06 +0700 (+07)

I want to note that the field can be without a domain, example:
(Authenticated sender: postmaster)

Thx

[zbateson]

Hi @toto4ds --

I specifically and purposefully left out other parts of a Received header, because there is so much variation and no clear guideline to follow, and because I don't want to chase differences in the wild. You can look at and parse those yourself though by calling $header->getComments() and looking for one that starts with "Authenticated sender:" or something.