Support direct filtering of received mails by user- and message-predicate

[marcelmay]

• Bypasses protocols such as IMAP or POP3 by directly accessing internal user and mail data
• Allows filtering user mail-boxes and filtering messages via predicates
• Example:

      // All messages received for emails ending with "localhost" and subject containing "match"
      Stream<MimeMessage> messages = greenMail.findReceivedMessages(
          u -> u.getEmail().toLowerCase().endsWith("localhost"),
          m -> MimeMessageHelper.getSubject(m, "").contains("match"));