Add support for OpenAPI

[transitive-bullshit]

Add support for creating an OpenAPI schema from an FTS Definition and URL.

This will allow us to easily solve #9 and #10.

https://github.com/wework/json-schema-to-openapi-schema looks useful.

[transitive-bullshit]

Useful links:

• https://github.com/swagger-api/swagger-codegen - Swagger codegen.
• https://github.com/OpenAPITools/openapi-generator - OpenAPI codegen.
• https://openapi.tools
• https://github.com/APIDevTools/swagger-parser - Validation.
• https://github.com/apigee-127/sway - Might be useful.
• OpenAPI Spec
• OpenAPI Guide

[transitive-bullshit]

Although it's not 100% tied to FTS directly, we solved this in a related project fin-to-openapi.