feat: Subscription executor has --override-result-topic option

[lynnagara]

Add an --override-result-topic option to the subscription executor.
Currently this is not wired up to anything but it will support
temporarily routing the produced subscription results to a different
topic so that the executor can be safely tested without affecting actual
users in production. The option should be removed once we are done testing.

[codecov-commenter]

Codecov Report

Merging #2281 (578afe6) into master (2a22e70) will increase coverage by 0.11%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #2281      +/-   ##
==========================================
+ Coverage   93.08%   93.20%   +0.11%     
==========================================
  Files         550      551       +1     
  Lines       24927    24990      +63     
==========================================
+ Hits        23203    23291      +88     
+ Misses       1724     1699      -25     

Impacted Files	Coverage Δ
snuba/cli/subscriptions_executor.py	64.28% <100.00%> (+1.32%)	⬆️
snuba/subscriptions/executor_consumer.py	88.98% <100.00%> (+0.59%)	⬆️
tests/subscriptions/test_executor_consumer.py	100.00% <100.00%> (ø)
tests/admin/test_api.py	100.00% <0.00%> (ø)
snuba/state/__init__.py	80.58% <0.00%> (+0.58%)	⬆️
snuba/admin/views.py	80.00% <0.00%> (+80.00%)	⬆️
snuba/admin/runtime_config/__init__.py	84.61% <0.00%> (+84.61%)	⬆️

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