Add support for 12Factor-style JSON logging

[mdekstrand]

This adds support for configuring the logger to write JSON logging to the terminal.

[codecov]

Codecov Report

Attention: Patch coverage is 84.61538% with 2 lines in your changes missing coverage. Please review.

Project coverage is 90.71%. Comparing base (f3e2274) to head (bcccbad).
Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
lenskit/lenskit/logging/tracing.py	77.77%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #591   +/-   ##
=======================================
  Coverage   90.71%   90.71%           
=======================================
  Files         106      106           
  Lines        6527     6529    +2     
=======================================
+ Hits         5921     5923    +2     
  Misses        606      606           

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