Config: Can add static headers to email messages

[owensmallwood]

fixes https://github.com/grafana/hosted-grafana/issues/5058

You can add static headers config in the smtp.static_headers section for headers to get added to emails.

Example grafana config could be:

#################################### SMTP / Emailing #####################
[smtp.static_headers]
Foo=value1
Bar=value2

Example
Add the following to your grafana config:

[smtp.static_headers]
Foo-Header=foo_val
Barheader=abc123

Edit: The header validation was updated to only work with smtp headers before I recorded this 👇
Then send an email and inspect the headers:

[owensmallwood]

@hairyhenderson Tested this locally with the smtprelay logging and it works as expected

[alexweav]

Overall this is looking great! I'll defer to the codeowners of the SMTP service for final approval, but this seems good to me. I had just a couple minor nitpicks.

[hairyhenderson]

LGTM! Thanks for working on this @owensmallwood 🥳