expose DD_CAPTURE_LAMBDA_PAYLOAD_MAX_DEPTH [SVLS-3853]

[joeyzhao2018]

What does this PR do?

1. Expose DD_CAPTURE_LAMBDA_PAYLOAD_MAX_DEPTH environment variable. This is to achieve feature parity. Universal instrumentation already enabled the env: https://github.com/DataDog/datadog-agent/pull/19450
2. Add a section in Readme file to serve as a doc for the environment variables introduced in this repo.

Motivation

Testing Guidelines

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)

[codecov-commenter]

Codecov Report

Attention: 118 lines in your changes are missing coverage. Please review.

Comparison is base (35295b3) 80.35% compared to head (8ed18b2) 81.27%. Report is 83 commits behind head on main.

:exclamation: Current head 8ed18b2 differs from pull request most recent head 9be5811. Consider uploading reports for the commit 9be5811 to get more accurate results

Files	Patch %	Lines
src/trace/step-function-service.ts	49.29%	35 Missing and 1 partial :warning:
src/trace/listener.ts	41.37%	17 Missing :warning:
src/trace/span-inferrer.ts	81.48%	0 Missing and 10 partials :warning:
src/trace/cold-start-tracer.ts	87.23%	2 Missing and 4 partials :warning:
src/trace/xray-service.ts	94.59%	5 Missing and 1 partial :warning:
src/trace/context/extractors/event-bridge-sqs.ts	61.53%	3 Missing and 2 partials :warning:
src/index.ts	90.47%	3 Missing and 1 partial :warning:
src/trace/context/extractor.ts	95.18%	4 Missing :warning:
src/trace/context/extractors/event-bridge.ts	63.63%	3 Missing and 1 partial :warning:
src/trace/context/extractors/kinesis.ts	69.23%	3 Missing and 1 partial :warning:
... and 9 more

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #430 +/- ## ========================================== + Coverage 80.35% 81.27% +0.92% ========================================== Files 36 50 +14 Lines 1761 2040 +279 Branches 410 467 +57 ========================================== + Hits 1415 1658 +243 - Misses 294 322 +28 - Partials 52 60 +8 ```

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