[fix] use X-Forwarded-Proto header if the URL scheme is unknown (#5106)

The HTTP X-Forwarded-Proto (XFP) request header is a de-facto standard header
for identifying the protocol (HTTP or HTTPS) that a client used to connect to a
proxy or load balancer.[1]

In our documentation[2] we recommend to set the `X-Scheme` header. This header
is not required if the `server.base_url` is set correctly.[3]

If none of these URL scheme details exist, then the header X-Forwarded-Proto is
evaluated as a third alternative.

[1] https://developer.mozilla.org/en-US/docs/Web/HTTP/Reference/Headers/X-Forwarded-Proto
[2] https://docs.searxng.org/admin/installation-apache.html#apache-s-searxng-site
[3] https://docs.searxng.org/admin/settings/settings_server.html

Closes: https://github.com/searxng/searxng/issues/5105

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