Automatic merge from submit-queue

OpenAPI / Swagger2 spec generation

This is alpha version of OpenAPI spec generation. Generated "/swagger.json" file (accessible on api server) is a valid OpenAPI spec with some warnings that will be fixed in next versions of spec generation. Currently it is possible to generate a client using this spec though I did not test the clients.

reference: #13414

**Release note**:

```release-note
Alpha support for OpenAPI (aka. Swagger 2.0) specification serves on /swagger.json 
```