Allow to configure secrets in alertmanager component (platform)

Allow to specify secrets/configmaps in alertmanager statefullset of cluster monitoring.

2. What is the nature and description of the request?
Allow to add secrets (or additionally configMaps) in alertmanager configuration of cluster platform monitoring.

3. Why does the customer need this? (List the business requirements here)

To configure a receiver that needs, for instance, a private CA or a basic authentication with "password_file":

REF: https://prometheus.io/docs/alerting/latest/configuration/#http_config

4. List any affected packages or components.

Alertmanager