Change the way the ebpf programs execute to one that allows us to input data from a network packet

[thatjiaozi]

With this way of executing a bpf program, we can read the data sent on the packet using skb_load_bytes_relative() helper function.

This also changes the interface of the ffi execute to receive a serialized proto instead, this makes things a bit more flexible.

[thatjiaozi]

Note: This pr is dependant of https://github.com/google/buzzer/pull/32

Let's review that one first