Adds p_values for absolute and relative comparisons

[danielkberry]

This PR adds p-values to the output of mozanalysis.bayesian_stats.compare_branches. P-values are calculated using the p-value - confidence interval duality: given a bootstrapped confidence interval (as calculated using the existing methodology), this code calculates the corresponding p-value. A validation notebook is available here

[codecov-commenter]

Codecov Report

Attention: 5 lines in your changes are missing coverage. Please review.

Comparison is base (c69380a) 76.31% compared to head (557fe3c) 76.83%.
Report is 1 commits behind head on main.

Files	Patch %	Lines
src/mozanalysis/bayesian_stats/__init__.py	88.63%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #198      +/-   ##
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+ Coverage   76.31%   76.83%   +0.52%     
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  Files          23       23              
  Lines         992     1036      +44     
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+ Hits          757      796      +39     
- Misses        235      240       +5     

Flag	Coverage Δ
py38	76.83% <88.63%> (+0.52%)	⬆️

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[danielkberry]

One open question is around downstream impacts. @mikewilli, what changes need to be made to shared data models to accommodate the new field?