feat: add clientless Pub/Sub emulator service

[cofin]

Summary\n\n- add the official Google Pub/Sub emulator with bundled gcloud lifecycle checks\n- cover topic, subscription, publish, pull, acknowledgement, cleanup, and xdist isolation without a Python client\n- run smoke validation inside the emulator to avoid redundant sidecars\n\n## Validation\n\n- serial provider tests: 11 passed\n- outer xdist provider tests: 11 passed\n- Ruff, mypy, pyright, slotscheck, and docs passed\n- selector: 1 provider job, 1 image pull\n- no Google client dependency or lockfile change\n\nPart of #150.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 74.00000% with 13 lines in your changes missing coverage. Please review.
✅ Project coverage is 3.09%. Comparing base (ba9c758) to head (69c8241).

Files with missing lines	Patch %	Lines
src/pytest_databases/docker/pubsub.py	74.00%	13 Missing ⚠️

Additional details and impacted files

@@                  Coverage Diff                   @@
##           feat/podman-support    #157      +/-   ##
======================================================
- Coverage                11.95%   3.09%   -8.87%     
======================================================
  Files                       24      25       +1     
  Lines                     2082    2132      +50     
  Branches                   169     172       +3     
======================================================
- Hits                       249      66     -183     
- Misses                    1798    2066     +268     
+ Partials                    35       0      -35     

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