Add script to generate RPC wrappers

A new script, “autotools/build-rpc”, will be used to generate code for
RPC client wrappers. This is done because “lib/rpc.py” contains lots and
lots of boilerplate code. Forthcoming patches will start converting
RPC wrappers.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>