Add outgoing proxy support to oCIS

## Is your feature request related to a problem? Please describe.
Many customer environments have no direct internet access which means that external hostnames cannot be resolved or connected to. Accessing the internet needs an outgoing proxy to be used in this case (e.g. squid or alike).
Currently oCIS doesn't have a proxy support. So we cannot configure this proxy to connect to the internet. The proxy would also be needed to reach our own systems like the IdP as the IdP is resolved through the external name which can (in such environments) only be resolved by the proxy.

## Describe the solution you'd like
For proxy support there are well-known environment variables `HTTP_PROXY`, `HTTPS_PROXY` and `FTP_PROXY` since many years. The support for those variables should be added to the software. See also: https://about.gitlab.com/blog/2021/01/27/we-need-to-talk-no-proxy/

## Describe alternatives you've considered
For our IdP we could implement a shortcut to communicate with it internally, but this solution will fail to work if the IdP itself is on the public internet and not within the operation center of a customer.

## Additional context
None