Improve adapter configuration story re: attribute generation

[douglas-reid]

Tracking bug:

At some point in the near future, we need to add the following capabilities:

• type-checking on inputs to Generate (perhaps figure out how to wire in populated struct of inputs, etc.)
• type-checking on outputs from Generate (perhaps pass in AttributeDefinitions for controlling outputs, etc.)
• validation that generation of an attribute value is not occurring in multiple places (proxy-mixer, mixer APAs, etc.)

When we are ready to tackle this issue, separate issues should be filed an assigned for the subtasks.

[geeknoid]

Please close if this is fully redundant with your ongoing APA work.

[guptasu]

@geeknoid this bug can now be closed since the sub items are already covered by #1518 and #1477