Introduce more usable public API

[MikeEdgar]

Currently, platforms integrating with smallrye-open-api make use of some combination of at least the following classes:

• io.smallrye.openapi.api.OpenApiConfig
• io.smallrye.openapi.api.OpenApiDocument
• io.smallrye.openapi.runtime.OpenApiProcessor
• io.smallrye.openapi.runtime.OpenApiStaticFile
• io.smallrye.openapi.runtime.io.OpenApiParser
• io.smallrye.openapi.runtime.io.OpenApiSerializer
• io.smallrye.openapi.runtime.scanner.AnnotationScannerExtension
• io.smallrye.openapi.runtime.scanner.spi.AnnotationScanner

Many of the classes provide overloaded static methods that makes it hard to make modifications without studying each client.

We should have a single unified public "builder" style API that users access to generate OpenAPI models and de-/serialize them to/from YAML or JSON. It is a goal to limit exposing internal smallrye-open-api types in this new API.