feat(agent): Add dogstatsd.enabled flag to disable it when using a standalone DogStatsD

[rymir]

What this PR does / why we need it:

We're running dogstatsd in kubernetes as a standalone daemonset using https://github.com/DataDog/datadog-agent/blob/main/Dockerfiles/dogstatsd/alpine/README.md as we want the statsd UDP port to be up and running as soon as possible when we boot new kubernetes nodes.

There's currently no obvious way in the helm chart to instruct the datadog agent not to use dogstatsd, and ensure that an 8125 UDP port doesn't get assigned to the agent container.

This is key when running a separate dogstatsd daemonset, both in host networking mode, so that the daemonset don't clash on port 8125 UDP, without having to resort to brittle workarounds, as redefining the dogstatsd port to a different, when we shouldn't be binding the container port at all.

Which issue this PR fixes

Fixes #1589

Special notes for your reviewer:

Checklist

• Chart Version bumped
• Documentation has been updated with helm-docs (run: .github/helm-docs.sh)
• CHANGELOG.md has been updated
• Variables are documented in the README.md
• For Datadog Operator chart or value changes update the test baselines (run: make update-test-baselines)