HTTP: transform custom HTTP headers

[krizhanovsky]

Web-server mode (#471) requires setting of custom HTTP headers. Cache-Control, Expires and other similar server headers can be generated on this basis only. The feature is also useful for various proxy cache use cases.

The new configuration option must be added:

    hdr_add <header name> <header value>

Configuration example from #471:

    vhost tempesta-tech {
            location "?" {
                proxy_pass heavy;
                cache_fulfill;
                hdr_add Cache-Control "no-cache";
            }
            index   index.html;
            root_db /opt/tempesta/db/www_tt.tdb;
            hdr_add Expires "Thu, 15 Apr 2020 20:00:00 GMT"
    }

Moreover, proxy mode also requires adjustment of arbitrary HTTP headers, see for example Nginx's proxy_set_header.

Don't forget to develop appropriate functional test and update Wiki.