Add highs backend (WIP)

[brynpickering]

Fixes #822

TODO

• create verbose string representations programmatically (highs does not use standard math expressions by default)
• fix infeasibilities in operate mode, likely caused by incorrect col/row deletion when updating params, but could also be due to some presolve/warmstarting issues

Summary of changes in this pull request

• New Highs backend interface

Reviewer checklist

• Test(s) added to cover contribution
• Documentation updated
• Changelog updated
• Coverage maintained or improved

[codecov]

Codecov Report

❌ Patch coverage is 26.29310% with 171 lines in your changes missing coverage. Please review.
✅ Project coverage is 93.48%. Comparing base (31300db) to head (2b3de5e).

Files with missing lines	Patch %	Lines
src/calliope/backend/highs_backend_model.py	25.55%	165 Missing and 4 partials ⚠️
src/calliope/backend/__init__.py	33.33%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #856      +/-   ##
==========================================
- Coverage   96.70%   93.48%   -3.22%     
==========================================
  Files          38       39       +1     
  Lines        4851     5081     +230     
  Branches      630      669      +39     
==========================================
+ Hits         4691     4750      +59     
- Misses         65      231     +166     
- Partials       95      100       +5     

Files with missing lines	Coverage Δ
src/calliope/backend/expression_parser.py	92.99% <100.00%> (ø)
src/calliope/schemas/config_schema.py	100.00% <100.00%> (ø)
src/calliope/backend/__init__.py	90.00% <33.33%> (-10.00%)	⬇️
src/calliope/backend/highs_backend_model.py	25.55% <25.55%> (ø)

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