Allow realms to integrate with an SMTP server

[whaught]

TL;DR

Currently we use the default firebase email flow for password reset.
If we wish to use email for a 2nd factor, controler/eliminate rate-limiting concerns, and customize invitations, we need to be able to send our own emails.

Design

Proposal

1. Use the golang SMTP package to send email from a custom template
2. Include a realm setting for integrating with an email server credential
3. Create a realm setting that includes a text template for inviting new users.
4. Replace our hacked SendVerificationEmail with the template and use the server SDK generate link (which has 10x quota).

Alternatives considered
Integrate directly with SendGrid APIs

Additional information
Once we have this capability we can consider using this for TOTP sent via email for 2nd factor auth along with Twilio to replace the SMS based auth.