push-gateway: add component and config

After requests from users this commits adds the push gateway component
to the cluster. For that adds a deployment that deploys a replica of the
prometheus gateway and then defines different resources like the
service and the ingress to access the push gateway and the
servicemonitor to scrape the metrics on it.

Testing:
- Installed locally via helm install.

JIRA Refs:
- MONIT-4040