feat(ops): add transactions_ro storage support

[onewland]

Add a read-only mode to transactions via a new transactions_ro storage.

It should be settable via a global flag or by a referrer list in snuba.settings.

https://getsentry.atlassian.net/browse/SNS-990

[codecov-commenter]

Codecov Report

Merging #2331 (03c98f3) into master (e41efa4) will decrease coverage by 0.13%.
The diff coverage is 79.13%.

❗ Current head 03c98f3 differs from pull request most recent head e79390e. Consider uploading reports for the commit e79390e to get more accurate results

@@            Coverage Diff             @@
##           master    #2331      +/-   ##
==========================================
- Coverage   92.75%   92.61%   -0.14%     
==========================================
  Files         559      562       +3     
  Lines       25736    25871     +135     
==========================================
+ Hits        23871    23960      +89     
- Misses       1865     1911      +46     

Impacted Files	Coverage Δ
snuba/clusters/cluster.py	93.10% <ø> (-1.77%)	⬇️
snuba/settings/settings_distributed.py	0.00% <ø> (-100.00%)	⬇️
snuba/settings/validation.py	65.51% <ø> (ø)
tests/clusters/test_cluster.py	100.00% <ø> (ø)
tests/utils/as_code_formatter.py	62.82% <62.82%> (ø)
snuba/clusters/storage_sets.py	100.00% <100.00%> (ø)
snuba/datasets/entities/transactions.py	100.00% <100.00%> (ø)
snuba/datasets/storages/__init__.py	100.00% <100.00%> (ø)
snuba/datasets/storages/factory.py	100.00% <100.00%> (ø)
snuba/datasets/storages/transactions.py	100.00% <100.00%> (ø)
... and 12 more

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[fpacifici]

Please fix the table name.
Now this must be merged after the change to the prod configuration to declare the storage set.

[fpacifici]

Thanks,
I guess #2334 goes first, right ? (or at the same time).