Support for openapi v3

[seandavi]

The openAPI 3 spec differs from swagger (openAPI v2).

• https://swagger.io/blog/news/whats-new-in-openapi-3-0/
• https://swagger.io/blog/api-strategy/difference-between-swagger-and-openapi/

I wanted to open this issue to discuss if/how rapiclient could/should support openAPI v3.

[daniel-fahey]

This can downgrade OpenAPI v3 to v2 https://github.com/LucyBot-Inc/api-spec-converter

[mrchypark]

@daniel-fahey even can downgrade version, I think it would better to support each version.