A Feature Probe API for cilium/ebpf

Currently bpftool as a userspace utility tool living in the linux kernel can do ebpf feature probing: bpftool feature probe. As the Go+ebpf user-base is growing, it would be useful to have the possibility to probe the kernel for available ebpf features from other userspace programs written in Go without having to shell out to bpftool. This proposal is about adding a feature probe API to cilium/ebpf that gives users a standardized interface to query the kernel for available ebpf features, furthermore encouraging the use of Go within a fast growing ebpf community.