Expose proxy support within GitProxy itself for air gapped environments

[coopernetes]

Is your feature request related to a problem? Please describe.
It's very common in organizations to have web proxies configured for developer machines for security. Currently, GitProxy itself does not expose a way to enable "proxying the proxy" - that is, when GitProxy forwards requests to the upstream destinations, it presumes direct Internet access with no ability to hook in http_proxy settings.

It is supported in the middleware by overriding the agent property in the reqOpt via the proxyReqOptDecorator function. See villadora/express-http-proxy#337 (comment) for details.

Describe the solution you'd like
A GitProxy user should be able to specify the appropriate HTTP proxy agent options via configuration to enable the git server to connect to a destination via an external proxy.

Describe alternatives you've considered
There are no other viable alternatives with the current middleware in use. The current design presumes that the GitProxy server itself has direct network access to upstream locations (https://github.com). For some organizations, this may not be compatible with their network security.

Additional context
This was reported by a user getting started with the project so it's best to include it in our introductory documentation.