Distinguish between DHIS2 API operations and resources

[cjmamo]

At our last DHIS2 milestone demo, @david-mackessy showcased the /merge API endpoint. This endpoint is actually an operation and not a resource. It would be useful in my opinion if we syntactically distinguish between an API resource and an operation. For example, in FHIR, API operations are prefixed with a dollar sign (e.g., /$merge). One of the first benefits I see is that there would be no ambiguity with what is a resource and what is an operation. Another benefit is that we can apply special semantics when the endpoint is known to be an operation (e.g., tools which generate clients from the OpenAPI definition can generate code for operations in a different way). It should be relatively easy to implement this style of naming in a backwards compatible way for existing endpoints.