feat: Add LLM Observability env vars

[avangelillo]

What does this PR do?

Add the LLM observability env vars in line with their description.

Docs: https://docs.datadoghq.com/llm_observability/sdk/?tab=nodejs#command-line-setup

Motivation

• Github issue: Add support for LLM Observability #446
• JIRA: https://datadoghq.atlassian.net/browse/SVLS-7054

Testing Guidelines

1. Deployed the example stack to the sandbox account with no changes
2. Implemented the change in datadog-lambda
3. Saw no diff when I ran cdk diff
4. Added the env vars to the datadog-lambda
5. Reran cdk diff to see the following changes

Stack CdkTypeScriptStack-alexangelillo
Resources
[~] AWS::Lambda::Function hello-node hellonode2FDD99B8
 └─ [~] Environment
     └─ [~] .Variables:
         ├─ [+] Added: .DD_LLMOBS_AGENTLESS_ENABLED
         ├─ [+] Added: .DD_LLMOBS_ENABLED
         └─ [+] Added: .DD_LLMOBS_ML_APP
[~] AWS::Lambda::Function hello-python hellopython5270C3B7
 └─ [~] Environment
     └─ [~] .Variables:
         ├─ [+] Added: .DD_LLMOBS_AGENTLESS_ENABLED
         ├─ [+] Added: .DD_LLMOBS_ENABLED
         └─ [+] Added: .DD_LLMOBS_ML_APP

6. Deployed and checked the env vars on the lambda

Additional Notes

The descriptions are copied directly from the other docs

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

[domalessi]

Just two points of feedback. Otherwise, lgtm!