v0.4.0: Full RTP header parsing

[tobbee]

Summary

• RTP header removal now parses the full header (CSRC entries, extension headers) instead of assuming a fixed 12-byte size
• Shared RTP header parsing logic between pcap-unpack and pcap-replay via internal/rtp.go
• Bumped version to v0.4.0

Test plan

• Run pcap-unpack on captures with standard 12-byte RTP headers
• Run pcap-unpack on captures with CSRC or extension headers
• Run pcap-replay and verify RTP stripping works correctly