New version of CrestApi library and required changes in IOVDbSvc

The CrestApi library was redesigned in order to have a more clear mapping with the OpenApi description. The new version offers more solid type checking of exchanged data via the introduction of specific Data Transfer Objects (DTOs) which hide the json serialization and deserialization aspects. This new version presents a generic interface of the client API with two implementations, one for redirecting http(s) calls to the server, and another using filesystem as a backend (CrestClient and CrestFsClient respectively). IOVDbSvc has been changed to use the new CrestApi.