Consolidate HTTP server instrumentations to use single bpf probe implementation

[MikeGoldsmith]

Currently we have separate bpf probes for each http server instrumentation (net/http, gorilla mux & gin-gonic). The bpf code is identical across each implementation with only the go usage attaching the probe to different user space functions.

This PR does the following:

• Create new root bpf directory and move all bpf headers into it
• Move the net/http server bpf probe code to new bpf directory
• Update generate Makefile target to also set the BPF_PROBES directory which instrumentations can referece
• Update gorilla mux and gin-gonic instrumentations to use generic http service bpf probe, including:
  • Update event struct to add ParentSpanContext field
  • Add missing offset fields
  • Setting the parent span context when converting events
  • Updating expected test json structure to validate a parent span context is being set correctly
• Remove gorilla mux and gin-gonic custom bpf probes

[MikeGoldsmith]

Gin e2e test is still flakey - it works locally. Could someone else try and run to see if they something different?

[MrAlias]

Please rebase these changes into internal/pkg (changed in #282). The pkg directory is going to be removed in #302 given that package is no longer used.

[MikeGoldsmith]

Closing as there's been lot of changes in the bpf code and project structurally which has meant this change isn't as needed as before.