Allow to provide Open API specification

[Kaliumhexacyanoferrat]

As the developer of a web service application, I would like to be able to serve a useful Open API specification file, so that my clients can use my services more easily.

Example

var apiSpecification = OpenApi.Create()
                              .Contact(...)
                              .License(...)
                              .Server("https://production.mydomain.com", ...);                                                  

Acceptance criteria

• The functionality is implemented in a new module
• All meta data supported by the spec can be added (such as servers or descriptions)
• The handler allows both JSON and YAML to be generated
• There an option to generate the spec automatically from the handler tree
• The generation works for both web services and inline handlers (basically anything that uses the reflection module)
• The solution is covered by acceptance tests
• The solution is documented on the website
• For now, advanced use cases (such as example responses) do not need to be implemented