Add custom HTTP header for CDN integration

[yosukehara]

CDN need to refresh cache-data, we need to implement the headers at leo_gateway.
In addition, we'll consider leo_gateway is able to plug in custom headers.

[mocchira]

We will take this approach.

[yosukehara]

We've adopted Nginx format to the configuration file of LeoFS Gateway for custom headers.
In addition, specify a configuration path in leo_gateway.conf.

## append below line to leo_gateway.conf
http.headers_config_file = ./conf/nginx.conf
location /bucket_a/static_private {
    expires    24h;
    add_header Cache-Control private;
}
location /bucket_b/static_public_frequently_changed {
    expires    5m;
    add_header Cache-Control public;
}

[mocchira]

Done IMPL/UT/IT and merged into develop branch.

[mocchira]

In expires, we can use a part of this syntax.
For now supporting time intervals are seconds, minutes, hours and days.