Add "net" source with basic eBPF implementation

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• Add a generic "net" source and event type. Attempting to use the source on any platform other than Linux results in a runtime error for now. The idea would be for other OS implementations to exist over time.
• Provide proof-of-concept Linux eBPF program using CO-RE safe APIs. This only handles outbound IPv4 connections for now, but can be extended to collect more information. The program itself should be usable back to Linux 4.17 kernels, but I'm using the new-style BPF link syscalls, which came along in Linux 5.7 (from 2020).
• Introduce Nix flake with necessary C/LLVM tooling. The LLVM/C toolchain is only needed when modifying the C program: .o files are automatically generated for both big-endian and little-endian architectures and are checked into version control, so casual contributors will not need this for making changes to reveald as a whole. For folks working on the C program, the Nix flake provides a consistent, reproducible environment for generating the .o files regardless of OS being used for development.

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I have manually tested this locally by starting reveald and then curl-ing example.com. FYI using this source does require running reveald with elevated privileges, as one might imagine.

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Closing in favor of #19.