Enable random port on --port 0

[FabijanC]

Usage related changes

• If user specifies --port 0, the OS finds a random port.

Development related changes

• --port 0 is used in BackgroundDevnet as default to prevent the race condition in free port acquiring that has started to arise in Improve integration testing (file organization, time, memory) #664.
• Use new dependency netstat2 to determine the port where BackgroundDevnet spawned.
• Change the way spawning success is tested in BackgroundDevnet:
  • Test until one port associated with spawn subprocess PID. Then perform healthcheck (as before).
• Remove code duplication in BackgroundDevnet::send_custom_rpc.

Checklist:

• Checked out the contribution guidelines
• Applied formatting - ./scripts/format.sh
• No linter errors - ./scripts/clippy_check.sh
• No unused dependencies - ./scripts/check_unused_deps.sh
• No spelling errors - ./scripts/check_spelling.sh
• Performed code self-review
• Rebased to the latest commit of the target branch (or merged it into my branch)
  • Once you make the PR reviewable, please avoid force-pushing
• Updated the docs if needed - ./website/README.md
• Linked the issues resolvable by this PR - linking info
• Updated the tests if needed; all passing - execution info