Preserve Flow<...> in returnType and add returnsFlow flag on RpcCallable

[MiletaA]

Subsystem
Compiler Plugin, Descriptor Core

Problem Description
Descriptors for RPC methods that return Flow<T> were exposing returnType.kType == T instead of the declared Flow<T>.
It is issue #523

Solution
Stop unwrapping Flow<T> to T in RpcStubGenerator, so the generated descriptor now keeps returnType.kType == Flow<T>.
Add val returnsFlow: Boolean to RpcCallable and implement it in RpcCallableDefault; the compiler plugin sets this flag whenever the callable’s return type is a Flow<*>.