feat: Allow to use AWS secretsmanager with managed external secrets

[bebold-jhr]

See also DataDog/terraform-aws-log-lambda-forwarder-datadog#49

What does this PR do?

Allow the use of an AWS secretsmanager with managed external secret.

Motivation

We wanted to use external secrets, but the documentation said that you can only store plaintext secrets.
Also these issues/PRs:

• Feature Request: Support AWS Secrets Manager external rotation terraform-aws-log-lambda-forwarder-datadog#49
• [SVLS-9488] fix(secrets): recognize apiKey field in JSON Secrets Manager secrets datadog-lambda-extension#1305

Testing Guidelines

A new test which checks that extraction of the API key works for all three cases

DD_API_KEY
apiKey
plaintext

Additional Notes

Previously created as #1179
Conflicts resolved.

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)
• This PR passes the unit tests
• This PR passes the installation tests (ask a Datadog member to run the tests)