Add top-level search defaults and make stopping behavior configurable

[mdekstrand]

This PR adds configurability to the tuner's stopping behavior, for both Optuna and Ray. It also adds tuning.defaults to the main configuration schema to allow overall defaults to be changed in the configuration file.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.57%. Comparing base (e622c4e) to head (21fb6a1).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1151      +/-   ##
==========================================
+ Coverage   89.54%   89.57%   +0.03%     
==========================================
  Files         258      258              
  Lines       17313    17315       +2     
==========================================
+ Hits        15503    15510       +7     
+ Misses       1810     1805       -5     

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