authorizer trace context

[joeyzhao2018]

What does this PR do?

Fixes DataDog/serverless-plugin-datadog#252 for Python
Ticket: SLS-2499, RFC

Motivation

This solution is essentially adding an extra inferred span that connects the current two traces, i.e. the authorizer function trace and the main function trace, into one trace.

Implementation

• When in the authorizer function, inject the necessary trace information and the timestamp information into the returned response
• When in the main function, extract the information injected during authorizer function execution and create an inferred span based on it.
• More Details and test evidence: google doc link

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)

[astuyve]

Couple small notes, otherwise great work here Joey!

[duncanista]

Looking really good, nice job. Thanks for refactoring! 🚀