Support output proxy configuration

[cmacknz]

The shipper needs to support configuration of proxies for the output via the output.proxy_url configuration key as described in https://www.elastic.co/guide/en/fleet/current/fleet-agent-proxy-support.html#_set_the_proxy_for_communicating_with_elasticsearch.

The shipper should also support proxy configuration via environment variables as described in https://www.elastic.co/guide/en/fleet/current/fleet-agent-proxy-support.html#host-proxy-env-vars. Note that these environment variables are supported via the Go net/http/httpproxy package.

[pierrehilbert]

@cmacknz / @leehinman could we close this one with the new shipper plan?

[cmacknz]

This needs to stay open but be associated only with the V2 Elasticsearch output feature, not the shipper as a whole.