HaikuDepot: Change communication mechanism with server for repos

Previously the desktop application would make a number of JSON-RPC calls
over HTTP to get the repositories. Now it will make a single call to get
the repositories and cache the result. This uses standard HTTP cache
signalling techniques and allows the server-side the ability to cache
the generated data as well. Note that the model classes and parse-
related classes are generated and may not be code-style compliant. They
are generated from JSON schema files in the server-side project.
Information about this as well as the python files used to generate the
C++ classes and headers are included in the server-side project.