Add negative sampling to interaction matrix

[mdekstrand]

This adds negative sampling to matrix relationship sets, in both weighted and unweighted versions.

[codecov]

Codecov Report

Attention: Patch coverage is 94.00000% with 3 lines in your changes missing coverage. Please review.

Project coverage is 91.24%. Comparing base (b9fdf76) to head (4f3a91f).
Report is 7 commits behind head on main.

Files with missing lines	Patch %	Lines
lenskit/lenskit/data/builder.py	50.00%	1 Missing ⚠️
lenskit/lenskit/data/dataset.py	97.72%	1 Missing ⚠️
lenskit/lenskit/random.py	75.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #631      +/-   ##
==========================================
+ Coverage   91.23%   91.24%   +0.01%     
==========================================
  Files         117      117              
  Lines        8081     8125      +44     
==========================================
+ Hits         7373     7414      +41     
- Misses        708      711       +3     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.