Make sending large eBPF events more memory efficient

[MatteoNardi]

Make sending large eBPF events more memory efficient

This fixes #73 by removing variable-length fields from the eBPF event payloads and adding them to a dynamic buffer, sent after the event.

• eBPF buffer management code has been added to bpf-common/include/buffer.bpf.h
• variable length fields have been replaced by indexes (start + len) of the buffer
  • struct buffer_index is used on the eBPF side
  • struct BufferIndex is used on the Rust side
• Converting a C type to Event::Payload, now requires also the buffer. The IntoPayload trait was introduced.
• We use bytes::Bytes for buffer management, this results in less data copying.
  • There's a copy when reading the PerfEvent (unavoidable with aya APIs)
  • There's a copy when converting the C-like struct to event::Payload
    • Ideally we could remove this by using Bytes inside event::Payload
  • We no longer make a copy of big buffers which are processed before being converted to Payload. For example the UDP message buffer, which is used for intercepting DNS, is copied only once, from kernel to user space. Previously it was copied 2 times.

This was the simplest solution I could envision for #73.

I have

• run cargo fmt;
• run cargo clippy;
• run cargo testand all tests pass;
• linked to the originating issue (if applicable).

[banditopazzo]

you can update bytes to "1.3.0" in bpf-common/Cargo.toml

[banditopazzo]

also in network-monitor probe seems there is no check on length before output