First part of ENG-10730.
Added a scaffolding for the new endpoint for getting blob sizes:
- Accessible only with service-to-service token
- Used POST HTTP method to be able to conveniently pass many blob hashes in request body, without worrying about urlencoding limits. Since this is a service-only endpoint, we can change it later at any time.
- Request is a list of blob hashes
- Response is a map blob hash -> blob size
Depends on D14644