Producer API v.1.1

[wajda]

Add support for Attribute dependency info.

[wajda]

The Producer model is logically split into the operation-level and the attribute-level lineage information.
Attribute-level lineage is represented by the optional "schemas" property:

"schemas": {
  "attributes": [...],
  "functions": [...],
  "constants": [...],
  "mapping": {
    "op1": ["attrID1", "attrID2", ...],
    "op2": ["attrID3", "attrID4", ...]
  }
}

The first three properties (attributes, functions and constants) contain expression definitions of the corresponding type (similar to the "operations" object design).
"mapping" property maps operation IDs to their output schema definitions, i.e. array of attribute IDs that forms the given operation output.
Attributes and functions can refer other expressions as their children. i.e. expressions that the given attribute or function depends on.