Configuration to add custom SMTP headers

<!-- Please only use this template for submitting feature requests --> **What would you like to be added**: The ability to add custom SMTP headers to emails sent by Grafana **Why is this needed**: Certain SMTP providers support custom manipulation of emails according to SMTP headers. For example, Postmark [allows enabling or disabling link tracking](https://postmarkapp.com/developer/user-guide/tracking-links) by setting the SMTP header `X-PM-TrackLinks: None`. I believe this is an inconsequential addition that would benefit some use cases. An initial implementation could allow us to set this on `grafana.ini` with a simple `additional_headers: [key]=[value]`.