Add command header and dry-run flag

[pvdz]

This PR

• Adds a command header (currently disabled by default, you can make it appear with SOCKET_CLI_SHOW_BANNER=1)
• Adds a --dry-run flag to all commands except cdxgen
• Tests the --dry-run flag for all commands, asserting their output
• Fixed a bunch of commands. Some had the wrong command in the config, some had inconsistent error handling, etc.
• Exit code for invalid args/flags is now 2 for all commands
• No longer auto-showing the help when input validation fails

[jdalton]

I'll merge and follow-up with a fix for the fails. No worries.