[upstream] helm chart improvements, configure alertmanager url

[jotak]

Description

• Add CRD config for AlertManager, used for upstream
• Add prometheus-stack dependency in helm instead of "vanilla" prometheus => this is the kube-native one, that manages the ServiceMonitor and PrometheusRule CRDs

Dependencies

n/a

Checklist

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• Is this PR backed with a JIRA ticket? If so, make sure it is written as a title prefix (in general, PRs affecting the NetObserv/Network Observability product should be backed with a JIRA ticket - especially if they bring user facing changes).
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[openshift-ci]

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[openshift-ci]

@jotak: The following test failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
ci/prow/e2e-operator	945505d	link	false	/test e2e-operator

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