fix(http): use proxy agent setup that supports proxy env variables with no_proxy

[jontze]

Fixes a bug introduced in #762, where the "no-proxy" environment configuration was ignored.

With this PR I moved from the usage of https-proxy-agent to proxy-agent. This was done as https-proxy-agent was configured to take the proxy url from the environment but the library itself doesn't handle NO_PROXY / no_proxy configuration.

proxy-agent on the other side, is sitting on top of the https-proxy-agent and http-proxy-agent library and decides if the proxy is used based on the request URL and the domains specified in the "no proxy" environment variables.

---

To test this

1. Set the configuration for your proxy in the usual environment variables HTTP_PROXY , HTTPS_PROXY, http_proxy, https_proxy, NO_PROXY, no_proxy.
2. You can set your custom repository download url in the generator-cli.repository.downloadUrl config
3. Append your domain to the no_proxy and NO_PROXY environment variables OR if you don't have a mirror at hand, just add the default domain specified in config.schema.json and ensure that you have a way to verify that the traffic went not through the proxy.
4. Execute the generator

⚠️ Define both variables! If you only set the upper case NO_PROXY variable, the lower case one will have presence!

ℹ️ The downloaded jar will be cached near the dist files of the generator cli (so usually in you node_modules), beside you patched the version like this in your package.json of your project, then it's not in your node_modules:

  "@openapitools/openapi-generator-cli": "file:../openapi-generator-cli/dist/apps/generator-cli"