Add RPC service support to the RX pipeline

This is the final feature.

This changeset slightly modifies the error handling behavior of udpardRxSubscriptionReceive such that the datagram is deallocated if the passed arguments are invalid, unless self is also invalid. This is needed to prevent memory leak when the function is invoked incorrectly. An alternative solution is to state it explicitly in the API that the caller is required to free the datagram memory if -UDPARD_ERROR_ARGUMENT is returned, but this convention is comparatively fragile. A new test is added for this.

UdpardRxRPC is renamed into UdpardRxRPCPort for clarity.

UdpardRxRPCDispatcher is extended with local_node_id to facilitate destination address check when accepting frames.

After this one is accepted, the next steps are:

The internal e2e test (I am currently working on it).
A demo application in C that also serves as a high-level integration test. My original plan was to make it interact with a PyCyphal script but I am now considering a slightly different approach where the interaction will be arranged with a simple bash script based on Yakut. Stay tuned.

OpenCyphal / libudpard

Add RPC service support to the RX pipeline #47