Support functions deployed to other namespaces than openfass-fn

[nitishkumar71]

Description

Support redirect functions deployed in different namespace

Motivation and Context

• I have raised an issue to propose this change (required)
  Closes Add "namespace" to FunctionIngress spec #68

How Has This Been Tested?

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Impact to existing users

1. follow documentation to install openfaas and ingress-operator with pro version
   https://docs.openfaas.com/reference/ssl/kubernetes-with-cert-manager/
   https://docs.openfaas.com/deployment/pro/
2. Deploy FunctionIngress Defined below

apiVersion: openfaas.com/v1
kind: FunctionIngress
metadata:
  name: nodeinfo
  namespace: openfaas
spec:
  domain: "8761-103-178-143-208.ngrok-free.app"
  function: "nodeinfo"
  functionNamespace: "staging-fn"
  ingressType: "nginx"
  path: "/(.*)"

3. A new ingress should be created with annotation nginx.ingress.kubernetes.io/rewrite-target: /function/nodeinfo.staging-fn/$1

4. Any request sent to https://<domain>/v1/nodeinfo/ should work correctly.

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I've read the CONTRIBUTION guide
• I have signed-off my commits with git commit -s
• I have added tests to cover my changes.
• All new and existing tests passed.