Kourier's `kourier-bootstrap` contains the bootstrap config as https://github.com/knative-sandbox/net-kourier/blob/main/config/200-bootstrap.yaml.

As it is a raw config file, it is difficult to customize the value when we use operator.
So, this patch supports `bootstrap-configmap` field to customize it.

Usage:
```
cat <<EOF | kubectl apply -f -
apiVersion: v1
kind: Namespace
metadata:
  name: knative-serving
---
apiVersion: operator.knative.dev/v1beta1
kind: KnativeServing
metadata:
  name: knative-serving
  namespace: knative-serving
spec:
  config:
    network:
      ingress-class: kourier.ingress.networking.knative.dev
  ingress:
    kourier:
      bootstrap-configmap: my-configmap
      enabled: true
```

Then, users can deploy their boostrap configmap with name `my-configmap` to use it.