[python] Append-mode pre-check logic

[johnkerl]

Issue and/or context:

Tracking issue: #1278

Changes:

• Create the Signature class which will be used for append-mode pre-check on an upcoming PR
• Unit-test that

Notes for Reviewer:

• This is a split-out PR -- created for mercifulness to reviewers -- as a line-count reduction from raw material on [WIP] [python] Append-mode sketching #1498.
• Question for @aaronwolen @mojaveazure : If an input file has raw and non-raw data do we want them both to be ingested? Or just one or the other? This is actually a design question -- which I'll also put in our tracking doc.

[codecov-commenter]

Codecov Report

Patch coverage: 93.98% and project coverage change: +0.11% 🎉

Comparison is base (ce57a9d) 91.30% compared to head (c84d114) 91.41%.

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Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1554      +/-   ##
==========================================
+ Coverage   91.30%   91.41%   +0.11%     
==========================================
  Files          30       31       +1     
  Lines        2552     2668     +116     
==========================================
+ Hits         2330     2439     +109     
- Misses        222      229       +7     

Flag	Coverage Δ
python	91.41% <93.98%> (+0.11%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files Changed	Coverage Δ
...ython/src/tiledbsoma/io/registration/signatures.py	92.98% <92.98%> (ø)
apis/python/src/tiledbsoma/_arrow_types.py	94.66% <100.00%> (+1.56%)	⬆️
apis/python/src/tiledbsoma/io/ingest.py	84.72% <100.00%> (-0.23%)	⬇️

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[thetorpedodog]

Overall the structure of this code is good; I do have some suggestions and a few concerns about the API right now, though.

[johnkerl]

@thetorpedodog ready for round two

[thetorpedodog]

Other than the few comments below, looks good to me style-wise. The logic seems reasonable but I’m not super familiar; you may wish to get another reviewer to give that a once-over (but not mandatory).

[johnkerl]

@aaronwolen @mojaveazure any final concerns before I merge? This is of course a multi-PR (at least 3-PR) WIP and I'm happy to take feedback before or after the merge ...

[johnkerl]

Merging -- happy to take belated feedback as a follow-on PR