Add option to the Datadog scaler to use the Cluster Agent as proxy

[arapulido]

⚠️ This is not yet ready to be merge, but wanted to get some initial feedback

Add option to the Datadog scaler to use the Datadog Cluster Agent as proxy to obtain the metrics, instead of calling the REST API directly.

This avoids duplication and also improves the rate limiting issue that the Datadog scaler currently has, as the Cluster Agent gets the metric values in batches.

Implements: #5355

Checklist

• When introducing a new scaler, I agree with the scaling governance policy
• I have verified that my change is according to the deprecations & breaking changes policy
• Tests have been added
• Changelog has been updated and is aligned with our changelog requirements
• A PR is opened to update our Helm chart (repo) (if applicable, ie. when deployment manifests are modified)
• A PR is opened to update the documentation on (repo) (if applicable)
• Commits are signed with Developer Certificate of Origin (DCO - learn more)

Fixes #

Relates to #

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed in 7 days if no further activity occurs. Thank you for your contributions.

[zroubalik]

/run-e2e datadog
Update: You can check the progress here