Feature: configuration based rest proxy support

[jmartin-tech]

Adds configuration based support for proxies to the rest.RestGenerator.

This extracts the proxy capability from @au70ma70n's PR #878

Verification

example rest_config.json, add any required configuration for your rest endpoint found here:

{
    "rest": {
        "RestGenerator": {
           "uri": "http://my_internal_llm_endpoint",
           "proxies": {
              "http": "http://localhost:8080",
              "https": "https://localhost:8443",
           }
        }
    }
}

• garak -m rest -G rest_config.json -p lmrc
• Verify test with a proxy if possible, consider a simple port forward for http proxy.

In theory this capability is possible to drive via environment variables as noted in the requests library docs, this PR enables a configuration based method for when environment variables are not available for the user to set, or instance if garak were to be wrapped in an executor service.

[jmartin-tech]

Note that this PR also suggest additional possible sensitive configuration details that may deserve warnings similar to #1049, the format for proxy endpoints can accept a basic auth proxy in a format like http://user:pass@10.10.1.10:1080 as noted in the library doc examples.

[leondz]

few minor points in comments, lgtm

[leondz]

good thinking

[leondz]

i guess maybe this isn't the strongest possible test we can do (we could check mock proxy logs), but validation shows that the code works (by tailing proxy logs)