fix(propagation): make baggage header extract case insensitive

[ZStriker19]

Following the WSGI spec, headers shouldn't be case sensitive, therefore baggage, like the other headers we extract, should not be case sensitive.
https://peps.python.org/pep-3333/#environ-variables: "(A reminder for server/gateway authors: HTTP header names are case-insensitive, so be sure to take that into consideration when examining application-supplied headers!)"

Checklist

• PR author has checked that all the criteria below are met
• The PR description includes an overview of the change
• The PR description articulates the motivation for the change
• The change includes tests OR the PR description describes a testing strategy
• The PR description notes risks associated with the change, if any
• Newly-added code is easy to change
• The change follows the library release note guidelines
• The change includes or references documentation updates if necessary
• Backport labels are set (if applicable)

Reviewer Checklist

• Reviewer has checked that all the criteria below are met
• Title is accurate
• All changes are related to the pull request's stated goal
• Avoids breaking API changes
• Testing strategy adequately addresses listed risks
• Newly-added code is easy to change
• Release note makes sense to a user of the library
• If necessary, author has acknowledged and discussed the performance implications of this PR as reported in the benchmarks PR comment
• Backport labels are set in a manner that is consistent with the release branch maintenance policy

[github-actions]

CODEOWNERS have been resolved as:

releasenotes/notes/case_insensitive_baggage_header_extraction-63167c492474da6f.yaml  @DataDog/apm-python
ddtrace/propagation/http.py                                             @DataDog/apm-sdk-api-python
tests/tracer/test_propagation.py                                        @DataDog/apm-sdk-api-python

[datadog-dd-trace-py-rkomorn]

Datadog Report

Branch report: zachg/make_baggage_extraction_wsgi_compliant
Commit report: 33a0176
Test service: dd-trace-py

✅ 0 Failed, 87 Passed, 1378 Skipped, 3m 45.43s Total duration (35m 2.1s time saved)

[pr-commenter]

Benchmarks

Benchmark execution time: 2025-01-09 15:35:51

Comparing candidate commit 33a0176 in PR branch zachg/make_baggage_extraction_wsgi_compliant with baseline commit 5581f73 in branch main.

Found 0 performance improvements and 0 performance regressions! Performance is the same for 394 metrics, 2 unstable metrics.