Only pull in tracing-subscriber for tests, examples, and CLI binary

[ZackPierce]

Makes the tracing-subscriber dependency optional and mentions it in the cli feature-set.

This means the oso library no longer has a hard tracing-subscriber dependency, in accordance with best practices for leaving tracing-output selection flexible until the point of executable construction.

[github-actions[bot]]

CLA Assistant Lite bot All contributors have signed the CLA ✍️ ✅

[ZackPierce]

I have read the CLA Document and I hereby sign the CLA

[gj]

Thank you @ZackPierce !