Add `cf pcap` command to facilitate multi-instance packet captures

Introduce a new cf pcap command that captures network traffic directly from running application container instances via SSH. This provides operators and developers with a native way to debug networking issues without needing to manually SSH into containers and perform the packet capture.

This is part of RFC0048.

Motivation

With the new cf-pcap feature in diego, app developers can perform packet captures themselves without privileged support from the operator (if enabled). Currently, this requires an understanding of the implementation details of these packet captures which should not be the case. The user should be able to perform packet captures through the same interface as all the other actions, hence a new CLI command. Since this a core feature of diego I opted for a built-in command as opposed to a plugin.

Proposed Changes

New command: cf pcap

cf pcap APP_NAME [--process PROCESS] [-i INDEX]
   [-o OUTPUT_FILE] [--filter FILTER] [--interface INTERFACE]
   [--snaplen SNAPLEN] [--skip-host-validation]

Flags:

Behavior:

• Requires an org and space to be targeted
• When -i is omitted, captures from all instances simultaneously
• Opens SSH connections to each target instance and runs /tmp/lifecycle/cf-pcap remotely
• Streams pcap-formatted data over SSH stdout, parses individual packets using gopacket, and merges packets from all instances into a single output .pcap file
• Capture runs until the user presses Ctrl-C, at which point SIGINT is forwarded to the remote processes and the SSH sessions are torn down after a 5-second grace period
• Displays the output file path and size on completion

To allow merging the packet streams the github.com/gopacket/gopacket dependency needs to be added. I intend to only use the pure Go parts (no cgo) to keep building / installing the CLI simple.

Start with RFC0048 and the proposed cf pcap command interface, then inspect the CLI's existing command and SSH patterns. Evaluate the pure-Go github.com/gopacket/gopacket dependency and the /tmp/lifecycle/cf-pcap stream behavior. Done means the documented flags, multi-instance packet merging, Ctrl-C handling, and output reporting work as described.