Prepares an HTTP request to the endpoint at base_url with the params map containing request parameters.

• endpoint is one of :search, :reverse, :lookup, :status, :details.
• params is a map (not a keyword list!) with the request parameters.
• base_url is the the base URL of the target Nominatim API server.

You can use this function directly if you want to bypass the more user-friendly delegate functions in the main ExNominatim module and define the params map directly. This function does not perform any validation of the validity of the keys in params for the selected endpoint, or of their respective values vs. the API endpoint's specification.