SMTP Proxy network filter in envoy

*Title*: *Add SMTP Proxy support in Envoy *

*Description*:
Currently Envoy does not have SMTP protocol support. There is a starttls transport socket available in envoy which will be used to support STARTTLS command of SMTP protocol. Our use case is the TLS termination on envoy for smtp traffic i.e. smtp traffic from client to envoy will be encrypted and from envoy to the upstream will be plain text. This filter can be made generic to support other use cases as well. e.g. simply relay STARTTLS and further smtp commands to the upstream.

I am already working on the first use case of TLS termination.Would like any suggestions on rest of the possible use cases to be added.

[Links:]
>https://www.rfc-editor.org/rfc/rfc3207.html