Add support for proxy in extension manager, via config.json

This PR refer to #6241. It allows to add a proxy connection string
in config.json wich will beused by the extension manager when
downloading package file.

To use, add a line to `config.json`, in the `config` section
named `proxy`:
`config: { ... , proxy: "http://user:password@addres: port" }`.

This feature is tracked by trello card [902](https://trello.com/c/SEV95fdK/902-support-for-a-proxy-in-extensions-installation)