Set a callback invoked per-request to dynamically build SWML. When set, the
static SwmlBuilder is ignored and the callback’s returned
SwmlBuilder is used instead.
Callback function that receives (queryParams, bodyParams, headers) and
returns a SwmlBuilder or Promise<SwmlBuilder>.
On GET requests, bodyParams is always an empty object ({}). The body is only
parsed on POST requests.
this — Returns the service for method chaining.