Provide default OpenAPI settings & definitions

[olivierlemasle]

Since #65, custom-metrics-apiserver serves an OpenAPI spec; however, it requires a configuration and the generation by the project using custom-metrics-apiserver of OpenAPI definitions using openapi-gen.

That leads to duplicated works, each implementation project having to generate its own OpenAPI definitions.

E.g.:

• test-adapter generates definitions here and configures OpenAPI here;
• prometheus-adapter generates definitions here and configures OpenAPI here;
• keda generates definitions here and configures OpenAPI here.

It seams reasonable to provide these generated OpenAPI definitions in custom-metrics-apiserver, along with default OpenAPI serving settings (and let it possible to customize it).

Any thought?

/kind feature
/assign