[Step Function] 3.1 Refactor code for getting Lambda config

[lym953]

Context of this PR series

Right now the Datadog Serverless CloudFormation Macro only supports instrumenting Lambda functions. This series of PRs makes it also support instrumenting Step Functions.

Instead of merging every PR into main branch, I'm going to merge them into a feature branch yiming.luo/step-function to:

1. respect the holiday production freeze
2. avoid releasing partial support for step functions

What does this PR do?

Moves the code for getting Lambda config from index.ts to lambda/env.ts.

Motivation

I'm going to add similar code (for getting config) for step functions. Putting it also in index.ts, as the Lambda code currently does, would make that file messy.

Testing Guidelines

1. Pass the existing automated tests in lambda/env.spec.ts

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