Ensure http.status_code is always a string

[DylanLovesCoffee]

What does this PR do?

Always converts the status code found in the event that invokes the Lambda to a string.

Motivation

APM expects that http.status_code should always be a string. If we find a statusCode/status_code field in the event that invokes the Lambda function, it can be an integer.

Testing Guidelines

Updated existing tests.

Additional Notes

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)