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 95.97315% with 18 lines in your changes missing coverage. Please review.

Project coverage is 90.04%. Comparing base (07f70c3) to head (6eab439).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
lenskit/lenskit/pipeline/builder.py	96.44%	9 Missing ⚠️
lenskit/lenskit/pipeline/_impl.py	94.36%	4 Missing ⚠️
lenskit/lenskit/pipeline/components.py	95.00%	2 Missing ⚠️
lenskit/lenskit/pipeline/config.py	89.47%	2 Missing ⚠️
lenskit/lenskit/pipeline/nodes.py	97.56%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #606      +/-   ##
==========================================
+ Coverage   89.98%   90.04%   +0.06%     
==========================================
  Files         107      108       +1     
  Lines        6617     6790     +173     
==========================================
+ Hits         5954     6114     +160     
- Misses        663      676      +13     

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