Support using HTTP proxies for connecting to provider APIs

[makkes]

In certain (private or corporate) environments usage of an HTTP proxy is mandatory to connect to services inside or outside of the environment. go-git-providers should support these use cases by consuming the HTTP_PROXY and HTTPS_PROXY environment variables.

[makkes]

From what I saw in manual tests was that those proxy variables are taken into account. I'll probably come up with a smoke test for each provider to make sure this behavior is supported by all providers.