* adds proxy_url support,
adds proxy_url to the dockerswarm, eureka, kubernetes and consul service discovery,
adds proxy_url to the scrape_config for targets scrapping,
http based proxy is supported atm,
https://github.com/VictoriaMetrics/VictoriaMetrics/issues/503
* fixes imports
* Add omitempty for DisableCompression and DisableKeepAlive fields in ScrapeConfig
* Add omitempty annotation to all the default/optional values
* Fix annotations after review