Handle a custom detail field in Lambda event payloads

[DylanLovesCoffee]

What does this PR do?

We don't check that the detail field in an event payload is a dict prior to calling .get() on it. Custom invocation events are common and so are generic names like detail so we should handle with care.

Motivation

#275

Testing Guidelines

Attempted to reproduce the issue with a target payload like:

{ "hello": "dylan", "detail": "{\"arn\": \"<aws.scheduler.schedule-arn>\", \"time\": \"<aws.scheduler.scheduled-time>\",\"id\": \"<aws.scheduler.execution-id>\"}" }

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)

[jcstorms1]

LGTM 👍

[astuyve]

Good work Dylan!