L2TPv3 AES Encryption

[lukego]

Add the capability to AES encrypt the payload of L2TPv3 packets with a static key.

• [ ] Sanity-check the idea and determine the most suitable variant of AES.
• [ ] Develop an AES Snabb app to encrypt and decrypt packets.
• [ ] Develop unit test (selftest function).
• [ ] Develop performance tests (benchmark script integrated with CI).
• [ ] Optimize the app until performance is satisfactory. (Consider enhancing DynASM to make the Xeon AES-NI offload instructions available.)
• [ ] Extend the snabbnfv configuration file with an optional static AES key to activate the encryption app.
• [ ] Add the configuration option to the Neutron API Extensions too.