chore: Refactor istio pkg

[c-pius]

Description

Changes proposed in this pull request:

• refactors the construction of the VirtualService object
  • moves the object construction out of the istio client
  • removes the dependency on the client for fetching the gateways and instead receives the gateways as an argument
  • => we have a pure function where output is fully dependent on input that is easily unit testable
  • added some guard clauses / sanity checks
• adds unit tests for the VirtualService construction
• refactors the construction of the HttpRoute object
  • added some guard clauses / sanity checks
• adds unit tests for the HttpRoute construction
• aligns methods in istio#client.go wrt. method name, argument name, return values
• moves the istio package from pkg to internal

Related issue(s)

• Preparation for Setting up proper owner references for VirtualServices managed by the KLM #1287

Examples of VirtualService objects

Before and After look equal ✅

Before

apiVersion: networking.istio.io/v1beta1                     
kind: VirtualService                                        
metadata:                                                   
  creationTimestamp: "2024-02-23T10:56:53Z"                 
  generation: 1                                             
  name: klm-kyma-watcher                                    
  namespace: kcp-system                                     
  resourceVersion: "86048"                                  
  uid: de1e8e9e-358e-46ad-a994-8795276ae55d                 
spec:                                                       
  gateways:                                                 
  - kcp-system/klm-watcher-gateway                          
  hosts:                                                    
  - host.k3d.internal                                       
  http:                                                     
  - match:                                                  
    - uri:                                                  
        prefix: /v1/lifecycle-manager/event                 
    name: kcp-system/klm-kyma-watcher                       
    route:                                                  
    - destination:                                          
        host: klm-event-service.kcp-system.svc.cluster.local
        port:                                               
          number: 8082                                      
                                             

After

apiVersion: networking.istio.io/v1beta1                     
kind: VirtualService                                        
metadata:                                                   
  creationTimestamp: "2024-02-23T10:54:20Z"                 
  generation: 1                                             
  name: klm-kyma-watcher                                    
  namespace: kcp-system                                     
  resourceVersion: "85740"                                  
  uid: 6abd22b4-bcce-4c99-848b-d022db6a2299                 
spec:                                                       
  gateways:                                                 
  - kcp-system/klm-watcher-gateway                          
  hosts:                                                    
  - host.k3d.internal                                       
  http:                                                     
  - match:                                                  
    - uri:                                                  
        prefix: /v1/lifecycle-manager/event                 
    name: kcp-system/klm-kyma-watcher                       
    route:                                                  
    - destination:                                          
        host: klm-event-service.kcp-system.svc.cluster.local
        port:                                               
          number: 8082       

[c-pius]

Removed versioned as the constructor doesn't include any special semantics with regards to version.

[c-pius]

Deleted the VirtualService on DEV and it got re-created as expected. Therefore moving it to "To Accept"