Enhance `Fetching a Service Instance` endpoint when ` instance_retrievable=false`

[gberche-orange]

Expected behavior

As an osb consumer
In order to discover existence of a service instance
I need to use the Fetching a Service Instance endpoint when instance_retrievable=false
And receive a 501 Not Implemented when the service instance exists
Or receive a 404 NOT Found when the service instance does not exist

Observed behavior

Currently a 404 is indifferently returned when instance_retrievable=false regardless of whether the related service instance exists

Currently

• given an existing service instance guid1
• given a service offering with instance_retrievable=false
• when the Fetching a Service Instance endpoint is queried
• then a 404 is returned
• and the following trace is displayed 2023-02-22T18:25:49.89+0100 [APP/PROC/WEB/0] OUT 2023-02-22 17:25:49.888 INFO 33 --- [nio-8080-exec-2] c.o.o.o.s.OsbCmdbServiceInstance : Unable to process ServiceBrokerRequest{platformInstanceId='null', apiInfoLocation='api.domain.org/v2/info', originatingIdentity=null', requestIdentity=null}GetServiceInstanceRequest{serviceInstanceId='guid1', serviceDefinitionId='d05f9707-1644-44dc-bc3f-567011650507', planId='2e60b4d0-162f-43f1-b2a6-21c002661ca7'}, caught: org.cloudfoundry.client.v2.ClientV2Exception: CF-ServiceFetchInstanceParametersNotSupported(120004): This service does not support fetching service instance parameters.

Affected release

Reproduced on version x.y
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[gberche-orange]

Rationale for this request, was lack of official support for fetch endpoint in https://github.com/cloudfoundry/cloud-service-broker/

The fork orange-cloudfoundry/cloud-service-broker@8fbb650 could have been rebased against upstream which has removed support for instances_retrievable=false ?