Add Py_NewInterpreterFromConfig() and PyInterpreterConfig to the Public API

[ericsnowcurrently]

In gh-98609 we added _Py_NewInterpreterFromConfig() and the _PyInterpreterConfig struct. We should consider adding them to the public API:

• drop the leading underscore from the names
• add entries to Docs/c-api/init.rst

This isn't an urgent need, since the API is unlikely to be in high demand. However, currently there isn't any way for users to currently configure a new interpreter (via Py_NewInterpreter()). This function will give them a supported way. Furthermore, PEP 684 ("A Per-Interpreter GIL") will likely specify that the two will be added to the public API.

The only question is if Py_NewInterpreterFromConfig() and PyInterpreterConfig are an appropriate (I hesitate to say "best") API for users. I think it is, given the simplicity, how easy it will be to add more settings in the future, and the parallel with Py_InitializeFromConfig()/PyConfig.

[ericsnowcurrently]

Note that, in gh-98609, we also added PyInterpreterState.feature_flags, several flag constants (Py_RTFLAGS_*), and _PyInterpreterState_HasFeature(). Since the "features" are administrated internally, there is little need to add those to the public API. We can always add them later if there's an actual need.

[nanjekyejoannah]

The APIs are now public, closing the issue.