Add support to POP3 protocol

[Logicbloke]

Use case

Adding support for the POP3 protocol.

Your idea for a solution

This would make maddy a comprehensive solution and a modern implementation that will effectively replace the dovecot/postfix duo that dominated the servers for quite some time.

• [X] I'm willing to help with the implementation

[henryaltman]

hope to see, do it, bro