Add support for "webhook" invocations from within Cloud Datum Stream expressions. In general, need to be able to perform an HTTP GET request, where the query parameters, HTTP Basic authentication, and arbitrary HTTP headers are provided via an expression function. The HTTP response can be limited to JSON.

For example, a httpGet(urlString, parameterMap, headerMap) could return a Map so the expression could then extract a desired value from the output. Each invocation could also be cached, to help optimise use across different expressions that might want to extract multiple values from a single response.

Some additional utility methods could make forming some headers easier, like a httpBasic(username, password) function for generating HTTP Basic Authentication header values.