Add Handlebars helper for resolving system and environment variables

I’m looking to stand up a standalone version for load testing. One of the needs is to be able to replace values in the response template from system or environment variables. Based on https://github.com/tomakehurst/wiremock/issues/789, looks like it’s not currently possible without repackaging the code, although I’m not sure even in that case how’d I register the custom helper class.

I can imagine such a helper to be pretty useful, and will be happy to make a PR. I’m not super familiar with Handlebars, but I think the following should get the job done:

{{var key='something' scope='[sys|env]'}}

If scope isn’t specified, the code will lookup in the system variables first, followed by environment variables. Any . or - in the key will be converted to a _ if scope=env, as well as uppercased. For example, key=wire-mock.rocks will be converted to WIRE_MOCK_ROCKS. No such translation will be performed for scope=sys.

[philippe-granet]

See PR #1085

[tomakehurst]

@zkendall I agree this would be useful, and I’m happy for it to be added. If you’re up for a creating a PR that would make things happen much more quickly.