Add NestedToMCMCAdapter to enable compatibility with MCMC (#2391)

[SamuelSonoiki]

Description

This PR addresses issue #2391 by adding the NestedToMCMCAdapter class to convert NestedSampler objects into an MCMC-compatible interface. This change ensures compatibility with tools like ArviZ for posterior analysis and visualization.

Key Changes:

• Implemented NestedToMCMCAdapter to reshape posterior samples from NestedSampler into a chain-and-draw structure expected by MCMC workflows.
• Added get_samples method to provide reshaped or flattened posterior samples.
• Added get_extra_fields method to simulate dummy sampling statistics such as acceptance probabilities, step sizes, and number of steps.
• Supports passing model arguments (e.g., data, labels) for models requiring input parameters.

This update allows seamless integration of NestedSampler results into ArviZ workflows and enhances usability for downstream analysis.

Checklist

• Follows official PR format
• Includes a sample plot to visually illustrate the changes (only for plot-related functions)
• New features are properly documented (with an example if appropriate)
• Includes new or updated tests to cover the new feature
• Code style correct (follows pylint and black guidelines)
• Changes are listed in changelog

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📚 Documentation preview 📚: https://arviz--2400.org.readthedocs.build/en/2400/