Migrate entity services model to OpenAPI 3.0

[grechaw]

Entity Services's vocabulary is derived from Swagger 2.0, which has morphed into OpenAPI.

The Open API 3.0 specification, in addition to having some improvements, also supports many more types, which was the main need we had for ES. If we upgrade, we stand a change to form a better foundation for many of our initiatives.

https://github.com/OAI/OpenAPI-Specification/blob/master/versions/3.0.0.md#dataTypes