add native SMTP client

[Francois Falala-Sechet]

Is your feature request related to a problem? Please describe.
We often need to send emails from our chatbots. Would be neat to have a native SMTP client in CSML, rather than having to create an app!

Describe the solution you'd like

do data = do email = {
    // required
    "from": "John <[e-mail]>",
    "to": "Jane <[e-mail]>",
    
    // either html or text is required, both can be set
    "html": "<h1>Hello in HTML!</h1>",
    "text": "Hi there in plain text",

    // optional
    "reply_to": "Iron Man <[e-mail]>",
    "bcc": "James <[e-mail]>",
    "cc": "[e-mail]",
    "subject": "Happy new year",
}

do SMTP(hostname) // mandatory
  .auth(username, password) // optional, defaults to no auth
  .tls(true) // optional, defaults to auto upgrade to TLS if available
  .port(465) // optional, defaults to 465
  .send(data) // mandatory to perform the request