Create endpoints for maatregelen and vereisten

[robbertbos]

Description

• Measures and requirements are in the task-registry (related to #64)
• Versioning is part #43

Technical implementation

• refactor existing instrument endpoints to:
  from instruments/ and urns/ to instruments/ and instruments/urn resp.
  NOTE: endpoints need to be adjusted in AMT.

• add new endpoints:

  • requirements (returning a list of urns of all requirements)
  • requirements/urn/{urn} (returning requirement with a specific urn)

• add new endpoints: measures and measures/urn

  • measures (returning a list of urns of all measures)
  • measures/urn/{urn} (returning measures with a specific urn)

• similar to how instruments are loaded upon start of server, requirements and tasks should be loaded upon start of server.