add: support setting proxy via custom env vars

[mgoetzegb]

What

Support setting proxy via custom env vars CSAF_DL_HTTP_PROXY and CSAF_DL_HTTPS_PROXY.

If set to a non empty string, they will take precedence over http_proxy and https_proxy and also their upper case versions.

Why

There are scenarios where we want to apply a proxy only for the CSAF Downloader. Setting http_proxy and https_proxy usually affects several other tools running in the same environment. So having the option to use the custom env vars allows us to avoid that.

References

VTI-619

Checklist

• Tests

[github-actions]

Conventional Commits Report

Type	Number
Added	1

🚀 Conventional commits found.

[mgoetzegb]

I removed the distinction

env var unset -> use http(s)_proxy env var if set
env var set to empty string -> disable proxy and ignore http(s)_proxy env var, even if it is set

Now it is in both cases: use http(s)_proxy env var if set

On second thought the distinction is probably a rather unexpected bahavior. And I don't see a strong need for such a feature to justify this. Especially in a kubernetes deployment etc. it is also not so easy to conditionally set an env var. Making it even less useful.