Clean up fields parameter choices in GET APIs

[sureshms]

Entities have two types of fields - attributes and relationships. The attributes are stored as part of the JSON document in the backend database for an entity. The relationship fields are not stored as part of the JSON document.

When an entity is retrieved in GET operation (either list entities or get a single entity):

• Setting attributes in an entity does not require additional database lookup and incurs no extra cost other than making the response payload larger. Hence, the fields param used for controlling the payload size typically does not include attribute field names. The exception to this rule maybe some large attributes of the type array.
• Setting relationships of an entity however requires a lookup into the entity_relationship table and hence has database lookup cost. Hence, the fields param typically includes relationship field names.

Following changes are proposed as part of this issue:

1. For API documentation purposes, a list of field names that can be passed as queryParam field that is a subset of fields in an entity is defined in every entity.
2. Any field name from an entity can be passed as a field name in queryParam. The default fields that are returned always in GET operations are ignored (and the entity will contain these fields). A field name that does not exist in an entity will result in an error indicating the field name is invalid.