Allow exogenous regressions in BayesianSARIMAX

[jessegrabowski]

Exogenous regressors are added via a time-varying observation bias to keep the statespace as small as possible.

We could consider allowing something fancier, but this is a good v0.

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Pull Request Overview

This PR renames BayesianSARIMA to BayesianSARIMAX and adds support for exogenous regressors in the model. The X suffix follows the standard convention for models with exogenous variables.

Key changes:

• Renamed the class from BayesianSARIMA to BayesianSARIMAX
• Added support for exogenous regressors via new parameters k_exog and exog_state_names
• Implemented exogenous data handling through observation intercept modifications

Reviewed Changes

Copilot reviewed 8 out of 9 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
pymc_extras/statespace/models/SARIMAX.py	Main model class renamed and exogenous regression functionality added
tests/statespace/models/test_SARIMAX.py	Updated tests to use new class name and added exogenous regression test
pymc_extras/statespace/core/statespace.py	Updated data handling for exogenous variables
pymc_extras/statespace/models/__init__.py	Updated imports and exports for renamed class
pymc_extras/statespace/__init__.py	Updated public API exports
Various other files	Updated references and documentation

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[Dekermanjian]

Jesse this looks great!