feat(pubsub-open-telemetry): add quickstart example

[alevenberg]

Part of googleapis/google-cloud-cpp#12531

[snippet-bot]

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• pubsub-open-telemetry/quickstart.cc:38, tag pubsub_publish_otel_tracing

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[alevenberg]

Addressed comments

[alevenberg]

Installed cmake format in a venv locally and ran using cmake-format pubsub-open-telemetry/CMakeLists.txt -o pubsub-open-telemetry/CMakeLists.txt