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: [Serverless] Capture lambda payload as tags datadog-agent#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.

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

Additional details and impacted files

@@            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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