Feature request: Allow disabling the server header entirely

[PokkaKiyo]

Greetings!

I would like to request the ability to disable the default server header entirely, as showing the default server: Granian header does raise some best practice issues for my projects.

I've already checked that there is a related PR #522, which would allow me to send a server header with some obscured value in every response, but I would like the ability to optionally not have the header be inserted at all.

For reference against other third party libraries, in uvicorn, this can be achieved with the command line option --no-server-header or environment variable "UVICORN_SERVER_HEADER"=0 (https://www.uvicorn.org/settings/#http)

[aldem]

Disabling server header would help in cases when the response is generated by Granian itself (some 400 responses).

This is really a bit uneasy as it is enough to generate a bad request to find out which server is behind the endpoint, as this request wouldn't even reach the app (and won't be logged, usually).

Probably even better - would be nice to be able to define which header should be sent if none is explicitly present in headers - it may be set to empty value to completely disable it.