feat: Add pcap receiver

[j4m3s-s]

Hey~

This PR (not ready for prod yet, I'd like to discuss some implementation details) adds support for PCAP receiver. I.e, reading a pcap file to replay it through goflow. (Issue #415 )

I have a few questions :

• by default reading a pcap uses libpcap (through google/gopacket) which is probably not something to ship by default.
  I've implemented a "stub" receiver that returns an error by default (with a build tag to build the pcap version). Wdyt ?
  If you agree with that approach, I can add documentation and build within the CI ~
• The daemon doesn't exit once it's finished reading (which I don't really find to be an issue, I just thought it would be worth mentioning)
• I'm quite conflicted into how I could add the choice of PCAP through the CLI.
  There are currently two adjacent things :
  • listening on a port (UDP) || a file for PCAP
  • choosing the "interpreter" of the data (i.e netflow, flow, etc)
    But currently there's only the listener argument which conflates the two in one.
    Do you have an idea / recommendation into which way you'd prefer me to implement ?

Thanks by advance,

[lspgn]

Thank you for your contribution.
Unfortunately, I do not have the time to maintain extra implementation nor test them.
It also introduces breaking changes with new function signature.
I initially assumed it would capture live interfaces instead of a file. GoFlow2 is not meant to be a batch tool. In this case I would suggest importing the "producer" library to convert the captured packets into protobuf or json.