Support for Multiple Email Clients

[ramanaditya]

• Database is already built in a view to use multiple email clients like, sendgrid, amazon ses, mailgun etc.
• But, on the Backend default connection is used which only uses the email client available in the env or settings
• Added the get_campaign_connection function which uses the db object to accept the email client details, default is still env or settings
• Used this function to get the new connection before sending mail