Some env variables depend on DD_TRACE_ENABLED

[joeyzhao2018]

What does this PR do?

Make some env variables depend on DD_TRACE_ENABLED, i.e. all False if DD_TRACE_ENABLED set to False:
DD_COLD_START_TRACING for cold start aws.lambda.import spans
DD_TRACE_MANAGED_SERVICES for inferred spans
DD_DECODE_AUTHORIZER_CONTEXT and DD_ENCODE_AUTHORIZER_CONTEXT for lambda authorizor spans (a special case for inferred spans)

Motivation

The features controlled by these env variables are all part of DD Tracing...

Testing Guidelines

Additional Notes

Types of Changes

• Bug fix
• New feature
• Breaking change
• Misc (docs, refactoring, dependency upgrade, etc.)

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• 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
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[astuyve]

I like the refactor of depends_on_dd_tracing_enabled here!