Avoid swallowing UndefinedFunctionError from GCS callbacks

[vku2018]

Summary

• detect optional GCS callbacks with Code.ensure_loaded? and function_exported?/3 instead of rescuing UndefinedFunctionError
• call exported callbacks directly so real undefined function failures inside gcs_object_headers/2 or gcs_optional_params/2 propagate
• add regression tests for callback bodies that raise UndefinedFunctionError

Fixes #30

Validation

• git diff --check
• Unable to run mix format --check-formatted or mix test because elixir/mix are not installed in this environment.