[RFC] pwru: Support --output-skb-metadata

[Asphaltt]

Fixes #502

I'm porting such feature from btrace's --output-arg 'skb->hash'.

---

It's really useful to output specified metadatas of skb instead of outputing the whole skb btf data.

However, it's not easy to --output-skb-metadata 'skb->mark' because the metadata is provided by users.

Here're the steps to achieve it:

1. Parse the text to C AST.
2. Convert the C AST to bpf insns with skb btf type info.
3. Inject the set_skb_metadata stub with the generated insns.
4. Output the metadata with the provided field's btf type info.

e.g.

$ sudo ./pwru --filter-func 'icmp_rcv' --output-skb-metadata 'skb->hash' --output-skb-metadata 'skb->sw_hash' --output-skb-metadata 'skb->l4_hash' --output-skb-metadata 'skb->mark' host 1.1.1.1
2025/03/19 16:27:18 Attaching kprobes (via kprobe-multi)...
1 / 1 [----------------------------------------------------------------------------------------------------------------------------------------] 100.00% ? p/s
2025/03/19 16:27:18 Attached (ignored 0)
2025/03/19 16:27:18 Listening for events..
SKB                CPU PROCESS          NETNS      MARK/x        IFACE       PROTO  MTU   LEN   TUPLE FUNC
0xffff991f3ec49600 6   <empty>:0        4026531840 0            ens33:2      0x0800 65536 64    1.1.1.1:0->192.168.241.133:0(icmp) icmp_rcv hash=0 sw_hash=0 l4_hash=0 mark=0
^C2025/03/19 16:32:56 Received signal, exiting program..
2025/03/19 16:32:56 Detaching kprobes...
1 / 1 [----------------------------------------------------------------------------------------------------------------------------------------] 100.00% ? p/s