Explicitly declare supported public APIs with '__all__'

[sungwy]

Feature Request / Improvement

Currently, we rely on the informal naming conventions (with or without underscore) to distinguish between a public or private API.

It will be helpful to declare the names of public APIs in the __all__ attribute on all our public API supporting modules as it will:

• control what APIs we expose to wildcard imports
• enhance discoverability of public APIs (when reading the source code as users or developers)
• and allow tools like Griffe to detect the correct public APIs for backwards compatibility analysis in our CI