Mail client migration

[vietj]

Attachements are now @VertxGen interface due to the support of payload streaming

[gaol]

I am taking this

[gaol]

I see this has been added: https://github.com/vert-x3/vertx-4-migration-guide/blob/master/asciidoc/modules/con_mg_changes-in-vertx-mail-client.adoc