Support generating OpenAPI specification in SE

[romain-grecourt]

Support Generating OpenAPI documents for Helidon SE based application.

Some ideas:

• custom set of annotations + annotation processor
• support MicroProfile OpenAPI annotations
• code introspection at build-time (complexity is questionable)
• exercise routing with a dummy WebServer implementation similar to io.helidon.webserver.testsupport.TestWebServer

Introspection is limited because of the programmatic model of SE. Hints are required from the user, we will need a way to specify additional metadata, with annotations or perhaps special objects passed to the routing.

E.g.

• what headers does an endpoint accept
• what response code are returned / when
• what entities are returned / when

Supporting the MicroProfile OpenAPI annotations could converge the MP and SE support. We could decide to support build-time only for Helidon SE.

[graemerocher]

You might want to look at https://github.com/micronaut-projects/micronaut-openapi/ for inspiration