Add hook to populate default attributes and generate method for ODS ops.
Previously default attributes were only usable by way of the ODS generated
accessors, but this was undesirable as
- The ODS getters could construct Attribute each get request;
- For non-C++ uses this would require either duplicating some of the default attribute generating or generating additional bindings to generate methods;
- Accessing op.getAttr("foo") and op.getFoo() would return different results;
The hook can be used to address these.
This merely adds this facility but does not employ by default on any path.