net: lib: http_server: Fix parsing of HTTP2 header frames with priority

In case priority flag is present in the HTTP2 headers frame header, we
should expect additional priority fields before the actual frame
content.

The stream priority signalling has been deprecated by RFC 9113, however
we should still be able to handle this in case some implementation
(nghttp for instance) sends them.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>