feat: [SVLS-5677] DynamoDB Stream event span pointers

[apiarian-datadog]

What does this PR do?

Adds Span Pointers for Lambdas triggered by DynamoDB stream events.

Testing Guidelines

Unit tests. Also checked with a live test lambda.

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]

Will update the PR once dd-trace-py 2.15.0 is actually released (probably next week)

[apiarian-datadog]

Confirmed that this works as expected and creates usable links with a live test lambda