rspamd protected endpoint

[casualmatt]

• I already searched past issues
• I already checked the wiki/TODO page

Is your feature request related to a problem? Please describe.

Haraka communicates with RspamD using the RspamD HTTP protocol, and it would be nice if we could make it easier to keep this connection safe by using HTTPs and Basic AUTH or any kind of header auth.

Describe the solution you'd like

The option to add HTTPs, and an additional header to the request to RspamD from the EnvVar.

Describe alternatives you've considered

I tried to customize the plugin myself, but my experience with node HTTP is quite limited, and I couldn't find a way around it.

[casualmatt]

@msimerson I found out how to solve it, and I will open a pr soon.