Improve support for multiple open API spec files

The endpoint /v3/api-docs now supports a subpath, like:
/v3/api-docs/{apispec} that will return that spec.

For example:

    @EnableOpenApiRouter(config = {
        "classpath:petstore.yaml",
        "classpath:other-openapi.yaml"})

Will have this endpoints:

• v3/api-docs -> will return petstore.yaml
• v3/api-docs/petstore -> will return petstore.yaml
• v3/api-docs/other-openapi -> will return other-openapi.yaml

This endpoint also support the resolve=boolean query param that will result in
a resolve of refs in the returned specs.