[APPSEC]: blocking for alb multi value headers events

[florentinl]

What does this PR do?

Currently, ASM returns the same response for all event types. It works in all cases except ALB with a target group that has the multi-value headers option turned on. In this case, headers must be sent in the multiValueHeaders field with type dict[str, list[str]] instead of in the headers field with type dict[str, str].

This PR fixes blocking for Appsec in the context of ALB events for lambdas in target groups with multi-value headers enabled:

• Add EventSubTypes for regular ALB and ALB multi-value headers to propagate the information
• Send a blocking response with multiValueHeaders instead of headers when required.
• [typo] renamed the sample events file

Motivation

While adding the ALB event types to the system-tests in : DataDog/system-tests#5181. I noticed that blocking responses did not work as expected in the multi-value headers case.

Testing Guidelines

• updated the unit tests
• the system-tests APPSEC_LAMBDA_BLOCKING scenario asserts that it works as it should:
  • in the job logs for the alb-multi event type, you can see all xpassed tests that were failing currently for blocking and are now passing
• manual testing: spinned up an ALB with multi-value headers and tested it.

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)

[joeyzhao2018]

LGTM