Extract trace context from SNS->SQS event when SNS has raw message delivery

[nhulston]

What does this PR do?

Adds support for extracting Datadog trace context from SQS message attributes when receiving messages from SNS with raw message delivery enabled.

Motivation

#579

Testing Guidelines

Tested manually with Lambda -> SNS -> SQS -> Lambda when SNS had raw message delivery enabled:

as you can see, both Lambdas and the SQS inferred span are on the same trace, but there is a missing SNS span (as expected).

Also added unit tests

Additional Notes

While we can extract the trace context and create the SQS inferred span, we can't create the SNS inferred span in this scenario since information is lost due to raw message delivery.

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)