Add method to fit Sandia inverter model using field data

[cwhanse]

• Closes #xxxx
• I am familiar with the contributing guidelines
• I attest that all AI-generated material has been vetted for accuracy and is in compliance with the pvlib license
• Tests added
• Updates entries in docs/sphinx/source/reference for API changes.
• Adds description and name entries in the appropriate "what's new" file in docs/sphinx/source/whatsnew for all changes. Includes link to the GitHub Issue with :issue:`num` or this Pull Request with :pull:`num`. Includes contributor name and/or GitHub username (link with :ghuser:`user`).
• New code is fully documented. Includes numpydoc compliant docstrings, examples, and comments where necessary.
• Pull request is nearly complete and ready for detailed review.
• Maintainer: Appropriate GitHub Labels (including remote-data) and Milestone are assigned to the Pull Request and linked Issue.

The method is described in a 2026 PVSC paper that is submitted but not yet published by IEEE.

I renamed the existing fit_sandia to fit_sandia_lab to distinguish from this new method fit_sandia_field. fit_sandia_lab should be used with data from the CEC laboratory test method.