feat: add due event fetcher and attribute enrichment

[JohanGHole]

Added:

• fetch due events route - fetches scheduled tracker events for today, given a map of configured program stages.
• fetch attributes route - fetches givenName and contactUrn attributes based on configured code values. Tracked Entities "owns" all Tracked Entity Attribute Values (Or "attributes" as described in the payload). However, Tracked Entity Attributes are either connected to a Tracked Entity through its Tracked Entity Type or a Program. Based on the DHIS2 configuration, we can therefore have either Tracked Entity Type Attributes or Tracked Entity Program Attributes. The importance of this separation is related to access control and limiting what information the user can see. In the test container, the attributes are configured to be represented as Tracked Entity Program Attributes, but in many real world cases, the attributes are Tracked Entity Type Attributes. The connector now supports both configurations. Not sure if an aggregation strategy is the best approach for this, as I am setting the endpoint for fetching the attributes dynamically (it can either be "tracker/enrollments" for Tracked Entity Program Attributes configuration, or "tracker/trackedEntities" for Tracked Entity Type Attributes configuration)

The next step will be to add the route for RapidPro contact creation based on the contactUrn , after which the event payload will be transformed to a RapidPro flow and added to a flow queue to be triggered in another route. In my first draft this was all located in the same file, but I will split it into separate files for better maintainability.