Allow referencing shared service instances via kyma CLI

[kwiatekus]

Description Kyma cli should make it easy to create 'reference-instance' that references already existing service instances. This way users get an open path to binding to existing instances and also deleting kyma environment w/o the need to remove all phisically provisioned service instances

Execution

• [x] try it out manually in btp (creating shared service instance and reference it ( and bind ) in kyma runtime) (1)
• [x] implement a command that would take service offering name and shared instance name to create a reference service instance

Reason Kyma CLI should enable users an effortless binding of service instances that were already created and marked as "shared" Cases for instance sharing:

• An application may consist of multiple microservices running on different platforms. Sharing the same RabbitMQ service instance would allow pubsub communication between those microservices. They can also share the same Postgresql service instance if needed.
• The application can be moved to a different platform and still use the same service instances that it previously used.

Attachment https://www.youtube.com/watch?v=AYULBewzEwI https://help.sap.com/docs/btp/btp-cli-command-reference/btp-share-services-instance https://wiki.one.int.sap/wiki/display/CPC15N/%5BCookbook%5D+Service+Instance+Sharing

[kwiatekus]

A target service instance must be "shared" <-- for now we treat this as a prerequisite, i.e user must share it via btp cockpit or btp cli

A kyma cli command to reference a shared instance in kyma runtime should create a service instance k8s resource (taking service offering name, and name of the shared instance from user input). It should use fixed reference-instance plan name

spec:
  externalName: object-store-ref
  parameters:
    referenced_instance_id: ''
    selectors:
      instance_label_selector: []
      instance_name_selector: object-store-cf
      plan_name_selector: ''
  serviceOfferingName: objectstore
  servicePlanName: reference-instance

and service binding

[kwiatekus]

 ./kyma-cli reference-instance --name-selector=kk-object-store --offering-name=objectstore --reference-name=kk-object-store-ref --kubeconfig=/Users/krzysztofkwiatosz/Downloads/cli.yaml

I would like to propose available options renaming:

• ~name-selector~ -> target-name-selector
• ~reference-name~ -> simply name

if kubeconfig not provided please default to value of KUBECONFIG env.

Is it possible to avoid requiring user to provide --offering-name=? can cli guess based on target instance?

[halamix2]

• with selector we would need to change all three to target-*-selector; in my option these flag names would be too long
• name looks OK
• for reading data from ENVs we have a working code snippet that uses VIper, it's a good idea
• There can be multiple instances of various types with the same name, so we would need to call BTP/CIS beforehand and look it up based on selectors/ID (if it is possible without knowing the offering name, we'd need to check)