update logging format

[FelixSchwarz]

Currently the delivery log (mailqueue.delivery_log) is pretty basic. After working with log files from msmtp and Exim, I would like to change the log format like this:

2024-08-18 11:53:08 recipients=bar@site.example from=sender@foo.example id=db11d8aa54d7053f86248173990fd5d3@site.example app=mq-sendmail host=mx.site.example smtpstatus=250 smtpmsg='250 OK id=1sfcsD-0000000010w-2ABZ'

Notes:

• use a single line per message (Exim uses two lines, one when it accepts the message and another when the outgoing message is delivered)
• key=value format
• SMTP response is helpful as it often contains an identifier for tracing on the SMTP server side but that might not be available in our SMTP client implementation right now.