Add S3 Downstream Span Pointers

[nhulston]

What does this PR do?

Adds span pointers to spans for Lambdas triggered by putObject, copyObject, and completeMultipartUpload events.

Span pointers are similar to Span Links, but for cases when it is impossible to pass the Trace ID and Span ID between the spans that need to be linked.

When the calculated hashes for the upstream and downstream lambdas match, the Datadog backend will automatically link the two traces together.

When clicking on the linked span, a new tab opens linking to the downstream Lambda function that was triggered by this S3 object update.

Motivation

This feature already exists in Python, and I'm working on adding it to all other runtimes (Node, .NET, Java, Golang).

Testing Guidelines

Easy: Checkout this span, enable the feature flag, and you will see that it's pointing to the downstream Lambda.

More thorough: Run this Lambda function with the event payload

{
  "shouldPutObject": false,
  "shouldCopyObject": false,
  "shouldMultipartUpload": false,
  "useV2": true
}

and change one of the bools to true. Also, try both with AWS SDK v2 and v3. Enable the span pointers feature flag, and check Datadog to ensure that the spans are properly linked.

Additional Notes

• This handles the 'downstream' case, for Lambdas that make S3 requests. This PR adds span pointers on the upstream case.
• S3 and Dynamo are supported in Python. I will soon open another PR to add support for Dynamo.

Types of Changes

• [ ] Bug fix
• [x] New feature
• [ ] Breaking change
• [ ] Misc (docs, refactoring, dependency upgrade, etc.)

Check all that apply

• [x] This PR's description is comprehensive
• [ ] This PR contains breaking changes that are documented in the description
• [ ] This PR introduces new APIs or parameters that are documented and unlikely to change in the foreseeable future
• [ ] This PR impacts documentation, and it has been updated (or a ticket has been logged)
• [x] This PR's changes are covered by the automated tests
• [ ] This PR collects user input/sensitive content into Datadog
• [x] This PR passes the integration tests (ask a Datadog member to run the tests)