Port direct SMTP sending support

[wleightond]

Canarytokens v2 supported a direct SMTP email channel. Currently v3 only supports Mailgun and SendGrid as mail providers, so we should port the direct SMTP functionality to v3 as well.

[emmanuel-thinkst]

Hey @wleightond

That would be fantastic, and hopefully we can better handle the SMTP issue that came up in canarytokens v2. (thinkst/canarytokens-docker#134)

Thanks