Fix error raised in the DCA when using external metrics

[davidor]

What does this PR do?

Fixes an error that happens when the external metrics option is enabled in the DCA:

  clusterAgent:
    config:
      externalMetrics:
        enabled: true

When creating a datadog metric, and an HPA object that references it, the DCA shows an error like this one:

...events is forbidden: User "system:serviceaccount:datadog:datadog-cluster-agent" cannot create resource "events" in API group ""...

This PR fixes the issue by adding create perms for events in the cluster agent cluster role. We're already setting these perms in the helm chart: https://github.com/DataDog/helm-charts/blob/0316b0ec18de7eda72279c3b683770efa502ffc7/charts/datadog/templates/cluster-agent-rbac.yaml#L22

Also note that I haven't tested the external metrics extensively, in this PR I just wanted to solve the RBAC issue.

Describe your test plan

Enable the external metrics option, create a datadog metric, then create an HPA object that references it, and make sure that the error above doesn't show in the cluster agent logs.
Docs about external metrics: https://docs.datadoghq.com/agent/cluster_agent/external_metrics/

[github-actions]

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[codecov-commenter]

Codecov Report

Merging #443 (df2b0d0) into main (96fa79c) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #443   +/-   ##
=======================================
  Coverage   61.05%   61.05%           
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  Files           3        3           
  Lines          95       95           
=======================================
  Hits           58       58           
  Misses         25       25           
  Partials       12       12           

Flag	Coverage Δ
unittests	61.05% <ø> (ø)

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