feat: implement service_instances and service_bindings endpoints

[kanngiesser]

Checklist:

• Have you added or updated tests to validate the changed functionality?
• Have you added Release Notes in the docs repositories?
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Outline

This PR adds the endpoints /v2/service_instances/:instance_id and /v2/service_instances/:instance_id/service_bindings/:binding_id which are defined by the Open Service Broker API Spec and updates the Service Catalog accordingly:

Service Catalog

• sets the property "instances_retrievable": true for each service offering
• sets the property "bindings_retrievable": true for each offering where "bindable" is true
  • (with the current implementation "bindable" is hard-coded to be true for all service offerings, though

/v2/service_instances/:instance_id

• returns offeringID, PlanID and provision parameters
• returns error codes according to OSB-API spec

/v2/service_instances/:instance_id/service_bindings/:binding_id

• returns only binding parameters
• returns error codes according to OSB-API spec

[linux-foundation-easycla]

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• ✅ login: kanngiesser / name: Jonas Kanngießer (46dc69b)

[kanngiesser]

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