TON-829: Accept Datadog Operator subscription

[fanny-jiang]

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What does this PR do?

Adds an EKS-only CloudFormation custom resource that discovers the public free Datadog Operator offer, validates its terms and zero-cost pricing, creates and accepts the AWS Marketplace agreement, and waits for the Operator entitlement. Existing active agreements are reused, and stack deletion retains the Marketplace agreement.

The custom resource uses the modeled boto3 Marketplace Agreement client for signing, response parsing, and bounded retries, plus the SDK paginator for entitlements. Deadline-aware calls and structured logs preserve actionable subscription failures without maintaining a custom HTTP client.

Motivation

Allow the EKS Quick Start onboarding flow to accept the Datadog Operator Marketplace subscription before the instrumenter installs the managed add-on, without requiring Marketplace permissions on the Datadog integration role.

Testing Guidelines

• Ran 147 Python unit tests covering subscription discovery, fail-closed free-offer validation, agreement creation and recovery, entitlement polling, deadlines, and CloudFormation responses.
• Verified the required Marketplace Agreement methods and entitlement paginator against boto3/botocore 1.43.44.
• Rendered the embedded Lambda source and validated the resulting template with cfn-lint.
• Ran bash -n on the release script and git diff --check.

Additional Notes

The repository uses master as its default branch; no main ref exists. The subscription resources are created only when EKS instrumentation is selected.

[gpalmz]

Left a handful of comments, mostly about the size of this. Two I'd want resolved before merge: the bundled boto3 question in the first one, and the GovCloud/China behavior.

[gpalmz]

Nice cleanup, this reads a lot better. Left two small deadline-handling things and one consistency nit, none blocking. Assuming the python3.14 boto3 probe comes back clean on the other thread.