Separate PipelineBuilder from Pipeline and update run api

[mdekstrand]

This makes several significant changes to the pipeline API:

• PipelineBuilder and Pipeline are now separate, and Pipeline is intended to be immutable.
• The run method is now takes a tuple instead of varargs, so that its return type can be more obvious.
• Config hashes are now properties.
• Default input connections are resolved at build time, not run time (and are no longer included in configurations).

Closes #604.

[codecov]

Codecov Report

Attention: Patch coverage is 89.89011% with 46 lines in your changes missing coverage. Please review.

Project coverage is 89.70%. Comparing base (07f70c3) to head (709ea7d).

Files with missing lines	Patch %	Lines
lenskit/lenskit/pipeline/builder.py	85.71%	36 Missing ⚠️
lenskit/lenskit/pipeline/_impl.py	93.97%	5 Missing ⚠️
lenskit/lenskit/pipeline/components.py	95.00%	2 Missing ⚠️
lenskit/lenskit/pipeline/config.py	87.50%	2 Missing ⚠️
lenskit/lenskit/pipeline/nodes.py	97.72%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #606      +/-   ##
==========================================
- Coverage   89.98%   89.70%   -0.28%     
==========================================
  Files         107      108       +1     
  Lines        6617     6816     +199     
==========================================
+ Hits         5954     6114     +160     
- Misses        663      702      +39     

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