config/schema: add schema and schema nodes ctors

It is the initial commit for the schema-aware data manipulation module.

Just add constructors here. Methods will be added in further commits.

The future config module will validate and manipulate hierarchical
configuration data. The schema module is a set of utilities for the
config module.

Part of #8725

NO_DOC=the module is for internal use from the config code
NO_CHANGELOG=see NO_DOC