Add "no-empty-http-proxy" linter

[leonklingele]

Your feature request related to a problem? Please describe.

The no-empty-http-proxy linter ensures instances of http.Transport always have their Proxy-field set in order to respect the system proxy settings (by using the HTTP_PROXY, et al. env vars)

https://github.com/stbenjam/no-empty-http-proxy

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Additional context.

No response