feat(schemas): Validate all source topics have schemas defined

[lynnagara]

There's a few temporary exclusions for schemas which still need to be created.

[codecov]

Codecov Report

Patch coverage: 95.65% and no project coverage change.

Comparison is base (20076d2) 90.72% compared to head (6ca0768) 90.72%.

❗ Current head 6ca0768 differs from pull request most recent head 4f13a3f. Consider uploading reports for the commit 4f13a3f to get more accurate results

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #4374   +/-   ##
=======================================
  Coverage   90.72%   90.72%           
=======================================
  Files         797      797           
  Lines       39175    39187   +12     
  Branches      245      245           
=======================================
+ Hits        35540    35551   +11     
- Misses       3603     3604    +1     
  Partials       32       32           

Impacted Files	Coverage Δ
tests/consumers/test_schemas.py	96.15% <90.90%> (-1.41%)	⬇️
snuba/admin/dead_letter_queue/__init__.py	96.77% <100.00%> (-0.73%)	⬇️
snuba/datasets/storages/factory.py	97.10% <100.00%> (+0.49%)	⬆️

☔ View full report in Codecov by Sentry.
📢 Do you have feedback about the report comment? Let us know in this issue.