Add extension operator registration flag.

[python-processing-unit]

Currently, only ASMODULE operators are restricted to the namespace of the extending module, and exposed to modules that import the extending module qualified by the extending module's name.

Add a new flag to operator registration. This flag MUST specify whether the operator is restricted to the the namespace of the extending module, and exposed to modules that import the extending module qualified by the extending module's name. Both ASMODULE and non-ASMODULE operators MUST NOT be be restricted to the namespace of the extending module (and so on) unless this flag is present.

Update both the implementation and the specification.