[SVLS-5265] S3 Event Handler Span Pointers

[apiarian-datadog]

What does this PR do?

Adding Span Pointers for the upstream S3 objects when a lambda is triggered by an S3 ObjectCreated event.

Motivation

This is required to support auto-span-linking when S3 objects are used to chain lambda functions together.

Testing Guidelines

Added unit tests.

Types of Changes

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

Check all that apply

• 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)
• This PR's changes are covered by the automated tests
• This PR collects user input/sensitive content into Datadog
• This PR passes the integration tests (ask a Datadog member to run the tests)

[apiarian-datadog]

This PR somewhat surprisingly passes integration tests. The S3 lambda handler spans do in fact include span pointers, but they're not being serialized in dd-trace-py in the extension-less integration test environment. The extension does ship these span links correctly, though.

We'll follow up with a patch on dd-trace-py to correctly include the span links and then our integration tests here will need to be updated accordingly: DataDog/dd-trace-py#10850