celbpf: evalute CEL expressions in BPF

[kkourt]

PRs:

• celbpf: evaluate CEL expressions in bpf #4504

TODO:

• docs
• support CEL expressions in tracepoints, uprobes, and USDTs (and multi-kprobe)
  • multi-kprobe support: celbpf: support multi-kprobes #4717
• more arithmetic operations: mult, div, shift
• fuzz testing over expressions
• helper functions such as ntoh
• support .data (in addition to .args)
• optimize code generation (either by using register-based code generation, or via peephole
• optimizations over the stack generated code)
• support member access (e.g., arg0.foo)