Move savings estimation to ESS and GSS, respectively

[paschai]

Summary: Moving the estimation methods to their respective base classes as part of cleanup: https://fburl.com/gdoc/zwgtrnn4

Also rewrote map_key setting logic in estimate_early_stopping_savings to be more readable

Differential Revision: D65840942

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This pull request was exported from Phabricator. Differential Revision: D65840942

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 81.25000% with 6 lines in your changes missing coverage. Please review.

Project coverage is 95.67%. Comparing base (2317f8c) to head (c0a7838). Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
ax/early_stopping/strategies/base.py	66.66%	4 Missing :warning:
ax/early_stopping/tests/test_strategies.py	85.71%	1 Missing :warning:
ax/global_stopping/strategies/base.py	80.00%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #3057 +/- ## ========================================== - Coverage 95.68% 95.67% -0.02% ========================================== Files 486 486 Lines 49304 49319 +15 ========================================== + Hits 47178 47187 +9 - Misses 2126 2132 +6 ```

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This pull request was exported from Phabricator. Differential Revision: D65840942

[facebook-github-bot]

This pull request has been merged in facebook/Ax@5f3981b38a2ab99679b420a4bd492fc5039ace30.