Feat: Use the lambda runtime as parent context through an opt-in flag

[rapphil]

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Description

In a previous PR #1657 the lambda behaviour was changed so that it extracs the context from the headers of the lambda event instead of from the lambda runtime, using the _X_AMZN_TRACE_ID env var.

This behaviour is undesireble as it is a breaking change to existing users.

This PR will add the previous behaviour through an opt-in flag so that users can be gracefully migrated or these two "modes" of operation can exist without conflict.

Type of change

Please delete options that are not relevant.

• [X] New feature (non-breaking change which adds functionality)

How Has This Been Tested?

Please describe the tests that you ran to verify your changes. Provide instructions so we can reproduce. Please also list any relevant details for your test configuration

• [x] Unit tests

Does This PR Require a Core Repo Change?

• [x] No.

Checklist:

See contributing.md for styleguide, changelog guidelines, and more.

• [x] Followed the style guidelines of this project
• [x] Changelogs have been updated
• [x] Unit tests have been added
• [x] Documentation has been updated