chart: Added PodDisruptionBudget for the loadtester

[Nerja]

This PR adds the ability to specify a PodDisruptionBudget for the loadtester similar to what we
have for the Flagger operator.

> helm template fl loadtester -f loadtester/values.yaml --set podDisruptionBudget.enabled=true
# Source: loadtester/templates/pdb.yaml
apiVersion: policy/v1beta1
kind: PodDisruptionBudget
metadata:
  name: fl-loadtester
spec:
  minAvailable: 1
  selector:
    matchLabels:
      app.kubernetes.io/name: loadtester
---
# Source: loadtester/templates/deployment.yaml
apiVersion: apps/v1
kind: Deployment
metadata:
  name: fl-loadtester
  labels:
    app.kubernetes.io/name: loadtester
    helm.sh/chart: loadtester-0.18.0
    app.kubernetes.io/instance: fl
    app.kubernetes.io/managed-by: Helm
...

[stefanprodan]

LGTM

Thanks @Nerja