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.
Can we avoid generating the method at all if we don't need to populate it?