fix(instrumentation): add OTEL_SEMCONV_STABILITY_OPT_IN to CLI args (#4438)

* fix(instrumentation): add OTEL_SEMCONV_STABILITY_OPT_IN to CLI args

OTEL_SEMCONV_STABILITY_OPT_IN was not registered in environment_variables.py
so the opentelemetry-instrument CLI did not expose a --semconv_stability_opt_in
argument. Add the constant so the auto-instrumentation argument parser picks
it up through the opentelemetry_environment_variables entry-point.

Fixes #3965

Signed-off-by: alliasgher <alliasgher123@gmail.com>

* chore: add CHANGELOG entry for #4438

Signed-off-by: Ali <alliasgher123@gmail.com>

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Signed-off-by: alliasgher <alliasgher123@gmail.com>
Signed-off-by: Ali <alliasgher123@gmail.com>
Co-authored-by: Mike Goldsmith <goldsmith.mike@gmail.com>
Co-authored-by: Aaron Abbott <aaronabbott@google.com>