feat(utils): Add helpers for circuit breaker and circuit breaker tests

[lobsterkatie]

This is a follow-up to #74557, which added the beginnings of a rate-limit-based circuit breaker, in the form of a new CircuitBreaker class. In this PR, various helpers, for checking the state of the breaker and the underlying rate limiters and for communicating with redis, have been added to the class. It also adds a MockCircuitBreaker subclass for use in tests, which contains a number of methods for mocking both circuit breaker and rate limiter state.

Note that though these helpers don't have accompanying tests, they are tested (indirectly) in the final PR in the series, as part of testing the methods which use them.

[codecov]

Codecov Report

Attention: Patch coverage is 42.85714% with 32 lines in your changes missing coverage. Please review.

Project coverage is 78.14%. Comparing base (7fc3a3d) to head (8609b63).

Additional details and impacted files

@@                           Coverage Diff                           @@
##           kmclb-add-CircuitBreaker-class-stub   #74559      +/-   ##
=======================================================================
- Coverage                                78.14%   78.14%   -0.01%     
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  Files                                     6731     6731              
  Lines                                   300065   300119      +54     
  Branches                                 51620    51636      +16     
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+ Hits                                    234495   234520      +25     
- Misses                                   59245    59276      +31     
+ Partials                                  6325     6323       -2     

Files	Coverage Δ
src/sentry/utils/circuit_breaker2.py	67.01% <42.85%> (-32.99%)	⬇️

... and 8 files with indirect coverage changes