First letter grouping

[mathemancer]

Fixes #408

This adds a grouping mode to the API which allows one to group by the first n characters of each entry in a column (after casting to TEXT if needed).

Technical details

n is given by the prefix_length parameter. Only works on single columns. In particular, this sorts out #408 for textual columns by giving "prefix_length": 1.

Also adds all GroupBy object properties to the returned grouping JSON object.

NOTE: This should be merged after #1312 .

Checklist

• My pull request has a descriptive title (not a vague title like Update index.md).
• My pull request targets the master branch of the repository
• My commit messages follow best practices.
• My code follows the established code style of the repository.
• I added tests for the changes I made (if applicable).
• I added or updated documentation (if applicable).
• I tried running the project locally and verified that there are no
  visible errors.

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[mathemancer]

This can be reviewed and merged as soon as #1312 is merged. Leaving as a draft until that point to avoid mistakes.

[codecov-commenter]

Codecov Report

Merging #1314 (ee686e1) into master (e2fedad) will increase coverage by 0.09%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1314      +/-   ##
==========================================
+ Coverage   93.49%   93.58%   +0.09%     
==========================================
  Files         114      114              
  Lines        4426     4491      +65     
==========================================
+ Hits         4138     4203      +65     
  Misses        288      288              

Flag	Coverage Δ
pytest-backend	93.58% <100.00%> (+0.09%)	⬆️

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

Impacted Files	Coverage Δ
mathesar/api/pagination.py	78.57% <ø> (ø)
db/records/operations/group.py	98.98% <100.00%> (+0.49%)	⬆️

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