RFC Support for Some Developer Utilities

On the Utilities for Developers page, it states:

Warning: These utilities are meant to be used internally within the scikit-learn package. They are not guaranteed to be stable between versions of scikit-learn. Backports, in particular, will be removed as the scikit-learn dependencies evolve.

If we want to provide utilities to support third-party estimators, we should treat some of these utilities as "first class" citizens.

For example safe_indexing would be extremely useful for third parties that want to support DataFrames as input. Currently, the options for third-party developers is to build their own "safe_indexing" or depend on our private version which may not be stable.

Another example is https://github.com/scikit-learn/enhancement_proposals/pull/22, which defines a n_features_in_ contract where we will internally use private methods to cohere with the contract. Third-party estimators would need to build their own methods or functions to work with the SLEP.

TLDR: Now that much of the utilities are "private", we can make deliberate decisions about what utilities should be public and supported by us. This would mean deprecation cycles, etc. If we support some of the utils module, it will make it easier to build estimators, which will enrich the ecosystem of scikit-learn compatible estimators.

CC @scikit-learn/core-devs

Start with the Utilities for Developers page and review the examples named in the issue, especially safe_indexing and the n_features_in_ contract from enhancement_proposals#22. Identify which utilities should become public, then define the supported API and deprecation approach for each; the issue is done when those decisions are documented and accepted.