authentication module to inspect http headers

[totaam]

For example, we should be able to enforce SSL.

The proxy can give us X-Forwarded-Proto and we should be able to reject http.

Related to #4437 and #4435.

[totaam]

Done in the commit above, defaults to checking for X-Forwarded-Proto=https, so:

xpra seamless --start=xterm \
    --bind-ws=0.0.0.0:10000,auth=http-header

Is equivalent to:

xpra seamless --start=xterm \
    --bind-ws="0.0.0.0:10000,auth=http-header,property=X-Forwarded-Proto,value=https"

Beware to only use this header check when the headers can be trusted!
(ie: if the xpra server is only reachable via a proxy server that will validate / set the headers in question)