Support importing schema from Open API spec

[igarashitm]

Please describe the feature that you want to propose

In the Open API spec file, it eventually refers to the data types defined in embedded JSON/XML schema.

As a first iteration, let user choose which message in the OpenAPI spec they want to use for the data mapping and render it.

cf. #1853
Ultimately, we would want to automatically detect if we want to create a request or read a response to/from the API and choose, but it needs to deeply analyze the camel route. That's for the future.