Add support to download documentation in OpenAPI 3.0

[rovellipaolo]

The serverless downloadDocumentation command currently supports to download the documentation only in OpenAPI 2.0 (Swagger).
It would be nice to support to (optionally) download the documentation also in OpenAPI 3.0.

See also:

• https://docs.aws.amazon.com/cli/latest/reference/apigateway/get-export.html
• https://docs.aws.amazon.com/apigateway/latest/developerguide/api-gateway-export-api.html

[ottopecz]

Resolved with v1.2.0