Index: support custom meta comparison function in diff

[pmrowla]

• Adds index.diff(meta_cmp_key=...) parameter
  • Can be used to specify a key function which will be used to compare old.meta and new.meta (rather than comparing the entire meta instance)
  • Works the same way as python sorted(key=...), i.e. meta_cmp_key=lambda m: m.etag can be used to only compare old.etag == new.etag in diff
  • When either or both old/new entry.meta is none, meta_cmp_key will be ignored
• additional kwargs for index.checkout() will be passed into diff() (only used when old index is also passed into checkout)

In DVC we want to avoid pushing a new version when the old and new checksums match, even if the version_ids do not match. i.e. someone else may have pushed a new version_id, but if their etag matches ours we do not need to push another duplicate copy, the "latest" version is still what we want it to be.

prerequisite for iterative/dvc#8409

[codecov-commenter]

Codecov Report

Base: 55.29% // Head: 55.21% // Decreases project coverage by -0.07% ⚠️

Coverage data is based on head (0395c6b) compared to base (0ee8097).
Patch coverage: 36.36% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #237      +/-   ##
==========================================
- Coverage   55.29%   55.21%   -0.08%     
==========================================
  Files          49       49              
  Lines        2930     2936       +6     
  Branches      501      504       +3     
==========================================
+ Hits         1620     1621       +1     
- Misses       1234     1237       +3     
- Partials       76       78       +2     

Impacted Files	Coverage Δ
src/dvc_data/index/diff.py	78.16% <16.66%> (-2.80%)	⬇️
src/dvc_data/index/checkout.py	61.81% <60.00%> (-1.65%)	⬇️

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