Add support for custom podLabels

This MR adds support for pod labels as done in the other Gitlab helm charts e.g. One should be able to customize through the values.yaml file

## Labels to be added to agent pod
##
podLabels:
  # Example:
  role: developer

# Source: gitlab-agent/templates/deployment.yaml
apiVersion: apps/v1
kind: Deployment
metadata:
  name: RELEASE-NAME-gitlab-agent
  labels:
    helm.sh/chart: gitlab-agent-1.3.0
    app.kubernetes.io/name: gitlab-agent
    app: gitlab-agent
    app.kubernetes.io/version: "v15.2.0"
    app.kubernetes.io/managed-by: Helm
spec:
  replicas: 1
  strategy:
    rollingUpdate:
      maxSurge: 0
      maxUnavailable: 1
    type: RollingUpdate
  selector:
    matchLabels:
      app.kubernetes.io/name: gitlab-agent
      app: gitlab-agent
  template:
    metadata:
      annotations:
        prometheus.io/path: /metrics
        prometheus.io/port: "8080"
        prometheus.io/scrape: "true"
      labels:
        app.kubernetes.io/name: gitlab-agent
        app: gitlab-agent
        role: "developer"

Closes #13 (closed)