Add support for return_components to irradiance.isotropic

[cbcrespo]

• Closes #xxxx
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• Tests added
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This PR is part of my GSoC 2026 project. One of the goals is to add support for return_components to all diffuse transposition models in pvlib, which currently is supported by some models (e.g. perez) but not others. This parameter allows the user to choose whether they want only the total diffuse irradiance (default), or are interested in the split between different diffuse components (sky diffuse, circumsolar, horizon brightening).

This has been implemented for irradiance.reindl in #2775.

For irradiance.isotropic specifically, return_components=True does not return additional information, but has been added in order to be consistent with other diffuse transposition models.

The docstrings have been changed accordingly, and a new test was added.

[AdamRJensen]

LGTM

[AdamRJensen]

Misssing a whatsnew entry fyi