Add support for SMTP configuration for api key based auth

[Shikhar-Nahar]

Currently the SMTP configuration is only possible via username andpassword authentication. However, using external service providers such as SendGrid requires login via apikey and setting the from mail to a specific id as whitelisted on the SMTP service provider.
Related to Issue#79
Currently it is not possible to configure SMTP to user SendGrid with the params allowed in
This blog gives the example for sending email using smtplib for SendGrid.

Which requires specifying the login user as 'apikey' while setting the from email to a specified whitelisted email.
The minimal required change for this feature would be to allow setting 'from' email in the email header as per user specified value.

smtp:
  email_mode: sendgrid  
  from_email: whitelisted-user@example.com
  login_user: apikey
  server: smtp.sendgrid.net
  port: 465
  password : <apikey>

and code in sender.py in herald to support above.

Tested with a workaround code like below which works

def _send_email_to_user_smtp():
    message.replace_header('From',from_email)
    try:
        with smtplib.SMTP_SSL(server, port) as smtp_server:
            smtp_server.ehlo()
            smtp_server.login(login_user, password)
            smtp_server.sendmail(from_email, message.get("To"), message.as_string())

[tguisep]

As workaround, finally I implemented an other solution - for invitation mainly - using a custom SMTP script and Optscale API to avoid code change.

[maxb-hystax]

Are you going to publish it in your fork? Maybe we'll be able to incorporate it into the code soon.