Allow covariates in plot_expected_purchases

[PabloRoque]

Description

Related Issue

• Closes #
• Related to #

Checklist

• Checked that the pre-commit linting/style checks pass. Feel free to comment pre-commit.ci autofix to auto-fix.
• Included tests that prove the fix is effective or that the new feature works
• Added necessary documentation (docstrings and/or example notebooks) using numpydoc format.
• If you are a pro: each commit corresponds to a relevant logical change

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📚 Documentation preview 📚: https://pymc-marketing--1430.org.readthedocs.build/en/1430/

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Codecov Report

Attention: Patch coverage is 20.00000% with 60 lines in your changes missing coverage. Please review.

Project coverage is 59.46%. Comparing base (542a85b) to head (60372e0).

Files with missing lines	Patch %	Lines
pymc_marketing/clv/models/beta_geo.py	21.12%	56 Missing ⚠️
pymc_marketing/clv/utils.py	0.00%	4 Missing ⚠️

❗ There is a different number of reports uploaded between BASE (542a85b) and HEAD (60372e0). Click for more details.

HEAD has 4 uploads less than BASE

Flag	BASE (542a85b)	HEAD (60372e0)
	11	7

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #1430       +/-   ##
===========================================
- Coverage   94.73%   59.46%   -35.27%     
===========================================
  Files          51       51               
  Lines        5505     5561       +56     
===========================================
- Hits         5215     3307     -1908     
- Misses        290     2254     +1964     

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[ColtAllen]

I'll review this in detail later, but it looks like there may be merge conflicts with #1390