Support http proxy in GitRepository/HelmRepository

[seaneagan]

If some of your sources need to go through a proxy while others do not, it could become difficult to manage the HTTP_PROXY / NO_PROXY env vars in the source-controller container appropriately. It would be useful to be able to specify a httpProxy directly in GitRepository/HelmRepository to avoid this.

[seaneagan]

... and ImageRepository

[hiddeco]

For the GitRepository type, you will likely run into the same shared runtime HTTP client issues as fluxcd/source-controller#93 (comment).

For the HelmRepository (and HelmChart) type, this will likely require upstream changes to the httpgetter.

[stefanprodan]

Yeah I see no way around the shared HTTP client in go-git, we need to fork it and try to port the changes upstream.

[gberche-orange]

The workaround of adding the http_proxy,https_proxy,no_proxy environment variables to the source controller deployment is not solving the use-case of selecting different http proxies per GitRepository/HelmRepository.

In our company, the workloads running on public cloud platforms need to select a distinct http_proxy for intranet access and internet access. This currently prevents some usage of GitRepository that require the non default http proxy.