APIGOV-32662 - add SetRevisionOnly to ServiceBuilder to skip instance creation for shared-ID proxies

[sbolosan]

When API Manager has multiple frontend proxies sharing the same API ID, we were creating a redundant APIServiceInstance for each one. Only one should own the instance.

To fix this, I added SetRevisionOnly() to the service builder so agents can flag a proxy as revision-only, meaning it publishes its spec but skips instance creation. The check is a simple early return inside processInstance. Added a getter IsRevisionOnly() on ServiceBody to keep it consistent with the existing pattern.

[jcollins-axway]

I do not see anything blocking the API Service update if revision only is set.

If the service does not exist we should create, but we should not update it at all.

[jcollins-axway]

See comment

[jcollins-axway]

if svc != nil && serviceBody.IsRevisionOnly() {
 serviceBody.serviceContext.serviceName = svc.Name
 serviceBody.serviceContext.serviceID = svc.Metadata.ID
 return svc, nil
} else if svc != nil {
 serviceBody.serviceContext.serviceAction = updateAPI
 httpMethod = http.MethodPut
 serviceURL += "/" + svc.Name
 c.updateAPIService(serviceBody, svc)
} else {
 svc = c.buildAPIService(serviceBody)
}