opentelemetry-instrumentation-aws-lambda: fix improper handling of header casing (#4216)

* opentelemetry-instrumentation-aws-lambda: fix improper handling of header casing

* update CHANGELOG.md

* add CIDict

* remove redundant generic superclass from CIDict

* fix formatting errors

* Update license comments to SPDX format

Replaced Apache License comments with SPDX identifier.

* Update license information in cidict.py

Removed Apache License comments and added SPDX identifier.

* add changelog fragment

* revert changelog modifications

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Co-authored-by: Mike Goldsmith <goldsmith.mike@gmail.com>
Co-authored-by: Aaron Abbott <aaronabbott@google.com>
Co-authored-by: Emídio Neto <9735060+emdneto@users.noreply.github.com>