Add support for API Key in SSM

[demolitionmode]

What does this PR do?

Adds option to fetch API Key from AWS SSM.

Motivation

I store my DD API Key as a SecureString in AWS SSM. Fetching from SSM is currently supported directly in the logs forwarder Lambda function, but it would be cleaner to support at this level so any future usage of this library has the functionality out-of-the-box.

Testing Guidelines

Additional Notes

Related MR for downstream lambda function (before refactor to use this library): DataDog/datadog-serverless-functions#196

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)