Add support for exogenous observation equations

[jessegrabowski]

Allow adding exogenous regressions to the obs_intercept term. This allows users to estimate stuff like trend directly inside the PyMC model.

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📚 Documentation preview 📚: https://gEconpy--57.org.readthedocs.build/en/57/

[codecov]

Codecov Report

❌ Patch coverage is 78.51240% with 26 lines in your changes missing coverage. Please review.
✅ Project coverage is 76.89%. Comparing base (da495b2) to head (c58ebde).

Files with missing lines	Patch %	Lines
gEconpy/model/statespace.py	78.51%	26 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #57      +/-   ##
==========================================
+ Coverage   76.41%   76.89%   +0.48%     
==========================================
  Files          33       33              
  Lines        4872     4987     +115     
==========================================
+ Hits         3723     3835     +112     
- Misses       1149     1152       +3     

Files with missing lines	Coverage Δ
gEconpy/model/statespace.py	83.54% <78.51%> (+5.68%)	⬆️

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