NM-266: add firewall mark flag to install cmd

[abhishek9686]

Describe your changes

Usage: netclient install --fwmark 51820 or netclient install -F 51820.
Defaults to 0 (disabled) when not specified, preserving existing behavior.

Provide Issue ticket number if applicable/not in title

Provide link to Netmaker PR if required

Provide testing steps

Checklist before requesting a review

• My changes affect only 10 files or less.
• I have performed a self-review of my code and tested it.
• If it is a new feature, I have added thorough tests, my code is <= 1450 lines.
• If it is a bugfix, my code is <= 200 lines.
• My functions are <= 80 lines.
• I have had my code reviewed by a peer.
• My unit tests pass locally.
• Netclient & Netmaker are awesome.