Support for POP3 SASL Plain

[wolftobias]

Currently the greenmail server doesn`t support the pop3 sasl auth plain command.
Similar like SMTP protocol, the pop3 variant of AUTH PLAIN has also a one line and a two steps mechanism.

1. CAPA must reply with "SASL PLAIN".
2. The POP3 server must understand a client send "AUTH PLAIN" command.
   2a. AUTH PLAIN <base64: username, authid, password>
   2b. AUTH PLAIN, S: +space, C: <base64: username, authid, password>

https://tools.ietf.org/html/rfc5034

6.  Examples

   Here is an example of a client attempting AUTH PLAIN (see [RFC4616])
   under TLS and making use of the initial client response:

        S: +OK pop.example.com BlurdyBlurp POP3 server ready
        C: CAPA
        S: +OK List of capabilities follows
        S: SASL DIGEST-MD5 GSSAPI ANONYMOUS
        S: STLS
        S: IMPLEMENTATION BlurdyBlurp POP3 server
        S: .
        C: STLS
        S: +OK Begin TLS negotiation now
            (TLS negotiation proceeds, further commands protected by TLS
            layer)
        C: CAPA
        S: +OK List of capabilities follows
        S: SASL PLAIN DIGEST-MD5 GSSAPI ANONYMOUS
        S: IMPLEMENTATION BlurdyBlurp POP3 server
        S: .
        C: AUTH PLAIN dGVzdAB0ZXN0AHRlc3Q=
        S: +OK Maildrop locked and ready

   Here is another client that is attempting AUTH PLAIN under a TLS
   layer, this time without the initial response.  Parts of the
   negotiation before the TLS layer was established have been omitted:

            (TLS negotiation proceeds, further commands protected by TLS
            layer)
        C: CAPA
        S: +OK List of capabilities follows
        S: SASL PLAIN DIGEST-MD5 GSSAPI ANONYMOUS
        S: IMPLEMENTATION BlurdyBlurp POP3 server
        S: .
        C: AUTH PLAIN
            (note that there is a space following the '+' on the
            following line)
        S: +
        C: dGVzdAB0ZXN0AHRlc3Q=
        S: +OK Maildrop locked and ready