Adding HTTP Proxy

[TBG-FR]

This PR aims to add the possibility to set a HTTP proxy, using environment variables. It answers to the following issue (and implemented a solution using the advice given here) : Is there a way to set proxy settings ?

So from now, you can set GLOBAL_AGENT_HTTPS_PROXY if you have proxy needs, and it will work 👌

Pull Request Process

• Ensure any install or build dependencies are removed before the end of the layer when doing a build.
• Update the README.md with details of changes to the interface, this includes new environment variables, exposed ports, useful file locations and container parameters.
• Increase the version numbers in any examples files and the README.md to the new version that this Pull Request would represent. The versioning scheme we use is SemVer.

Additionnal notes

The solution implemented uses 'global-agent, but it looks like it didn't need a npm install global-agentto add it to thepackage.json, I suppose it's included in another lib used by the package (maybe got` ?). If you want me to add it "in case of", I can do that !

Thanks in advance for checking that PR 😄

closes #2014

[jdalrymple]

Amazing, ill give it a look to double check the functionality and its limits!

[TBG-FR]

This needs further work. I finally stopped using gitbeaker and did my Gitlab API requests using "raw" node/ts for now.

When adding a proxy, gitbeaker's request (=> got requests) are facing a HTTP 403 Forbidden error which I couldn't solve. It works using fetch/got or even cURL, so I believe the issue lies in gitbeaker's code 👀 TODO

[twinkle77]

Global-agent may affects global code ？
What if I just want Got use a proxy
@TBG-FR

[jdalrymple]

Recently switched things to native fetch, is this functionality supported through the fetch api in any fashion?