API: Include Finch backend [draft]

[mtsokol]

Hi @hameerabbasi!

This PR is meant to incorporate Finch backend into pydata/sparse. My first idea is to create separate submodules for backends, pydata + finch, and select them based on an environment variable.

Finch backend could be installable via pip install sparse[finch].

Let's start working on this!

[codecov[bot]]

Codecov Report

Merging #645 (44e3256) into main (cb6b604) will decrease coverage by 74.04%. Report is 1 commits behind head on main. The diff coverage is 72.58%.

:exclamation: Current head 44e3256 differs from pull request most recent head 1e2b9a0. Consider uploading reports for the commit 1e2b9a0 to get more accurate results

Additional details and impacted files

@@             Coverage Diff             @@
##             main     #645       +/-   ##
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- Coverage   90.21%   16.18%   -74.04%     
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  Files          20       22        +2     
  Lines        3670     3714       +44     
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- Hits         3311      601     -2710     
- Misses        359     3113     +2754     

[hameerabbasi]

Superceded by #646 and #647.