Allow alertmanager alert routing to email/webhook

[samiulsami]

Allows Alertmanager to fan out alerts to email/webhook as configured in the values.yaml

Example:

helm:
  releases:
    opscenter-features:
      values:
        helm:
          releases:
            monitoring-operator:
              values:
                alertmanager:
                  email:
                    enabled: true
                    to: user+alertmanager@domain.com
                    from: user@domain.com
                    smarthost: smtp.domain.com:587
                    authUsername: user@domain.com
                    password: <your app password>
                    requireTLS: true
                    sendResolved: true
                  webhook:
                    enabled: false
                    url: ""
                    sendResolved: true

For Gmail testing:

• set smarthost to smtp.gmail.com:587
• enable 2-Step Verification on the sender account
• generate a Google App Password (https://myaccount.google.com/apppasswords) and use that as authPassword

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Requires these PRs:

• Allow alertmanager alert routing to email/webhook open-viz/grafana-tools#190
• Allow alertmanager alert routing to email/webhook open-viz/installer#262