Allow setting container registry used by cluster agent admission controller for library injection

[gaffneyd4]

What this PR does / why we need it:

I was reviewing the Admission Controller documentation regarding library injection for application instrumentation and came across this warning:

Docker Hub is subject to image pull rate limits. If you are not a Docker Hub customer, Datadog recommends that you update your Datadog Agent and Cluster Agent configuration to pull from GCR or ECR. For instructions, see Changing your container registry.

I checked the helm chart and it doesn't provide an option for doing this. This PR adds an option to the helm values (clusterAgent.admissionController.autoInstrumentationContainerRegistry) so that the environment variable DD_ADMISSION_CONTROLLER_AUTO_INSTRUMENTATION_CONTAINER_REGISTRY can be set in the datadog cluster-agent.

This PR solves this issue: https://github.com/DataDog/helm-charts/issues/1078

Checklist

[Place an '[x]' (no spaces) in all applicable fields. Please remove unrelated fields.]

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

[gaffneyd4]

Closing as this work has been duplicated by https://github.com/DataDog/helm-charts/pull/1376