Support instance sharing

[EpsilonTal]

Motivation

Sharing a service instance between different scopes allows apps in different scopes to share databases, messaging queues, and other types of services. This enables development teams to share a resource, while eliminates the use of service bindings as keys and user-provided services or other means to maintain credentials of instances that were provisioned in another scope.

Approach

• Support new properties of "shareable" and "referenced_instance_id"
• Setup new hierarchy for shared instance and its references
• Switch reference "instance_id" with its "referenced_instance_id"
• Broker can set plan as shareable
• Manage visibilities of the reference plan on sharing new instance of a service

Pull Request status

• [x] Initial implementation
• [x] Unit tests
• [x] Integration tests

[CLAassistant]

All committers have signed the CLA.