Add flags to subnet join

[dasconnor]

We should be able to run these commands in manual and automatic mode without any prompts. We need to add the flags to do so. We definitely need a --fuji and --mainnet. Then if the user provides either print or (--plugin-dir AND avalanchego-config) we skip the confirmations.

[holisticode]

For this to work we also need the flags --skip-whitelist-check and --force-write (the former for both situations described above, the latter only for the second, to edit the file)