You can create a configuration sort of entity with url attribute of string type for the end user to provide input url. You can retrieve this configuration in a microflow and extract url attribute and consider it for the REST API activity.
We had a similar requirement and to make it dynamic, we invoked ODATA service endpoints using REST activity. However, to leverage the metadata of entities and attributes, we had to build a generic interface that allows the end users to choose their entities and required attributes. Our solution does not handle association.