Implement object specs

[strogonoff]

Using objects specs, extensions will define how objects in a dataset should be parsed into runtime structured data representation. E.g., for YAML files, this would involve decoding a blob into text with utf-8 encoding, and deserializing resulting YAML string into a JS object.

• Implement object spec types
• Update extension maker API to support object specs
• Define signature of methods for querying index data to be made available in dataset context