collect-wsl-logs.ps1 should warn users when a required tool is not installed

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**Is your feature request related to a problem? Please describe.**
WSL developers need complete logs in order to investigate issues. Certain logs can only be collected when proper tools are installed on Linux.
It's common for people to miss these details when reading the documentation. Others don't read docs at all. This can result in incomplete logs and prevent issues from being root caused.

**Describe the solution you'd like**
It would be very useful for the script that collects the logs to check if the required tools are installed and write a clear warning in case some are missing, making it clear to the user that they should seriously consider installing the tool.

**Describe alternatives you've considered**
An alternative considered would be failing the script's execution when a tool is missing, but that is probably too drastic.
There might be a legitimate reason for the tool to not be installed, and blocking the collection of the other logs would arguably be worse.

**Additional context**
Suggested warnings:
WARNING: tcpdump not found on WSL. For a more complete log collection install tcpdump.
WARNING: iptables not found on WSL. For a more complete log collection install iptables.

Start by reading and running collect-wsl-logs.ps1 to see how Linux tools are currently used during collection. Check the tcpdump and iptables paths, then verify that missing tools produce clear warnings while the remaining logs are still collected. Done means users are warned about each missing required tool without the script failing.

Written by the indexing model from the issue text.