[Serving] Support for graph step input/result redirection

[yaronha]

storey steps usually receive the entire event and return the entire event, it accepts input_path and result_path parameters to control which portion of the event goes into the step, and where is the result written to allowing for greater flexibility.

This PR add support of input_path and result_path and add the same behavior to sync engine and to remote steps.

Usage example:

def mul(x):
     return x*2

function = mlrun.new_function("multiply", kind="serving")
flow = function.set_topology("flow")

# function input will be event["x"] and result will be written to event["y"]["z"]
flow.to(handler=mul, name="x2", input_path="x", result_path="y.z").respond()

server = function.to_mock_server()
resp = server.test(body={"x": 5})

# expect y.z = x * 2 = 10
# resp == {"x": 5, "y": {"z": 10}}