Prepare and execute RPC call, prepare result for the call. These
procedures are generic in the sense that they only require the
Call and Result types to be connected.
We use curl library for sending http requests for RPC; as the library's
usage can be disabled, we need to use CPP preprocessor macros for some parts
of the code.

Signed-off-by: Agata Murawska <agatamurawska@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>