`momento token generate` CLI command

[bruuuuuuuce]

• must write token + metadata somewhere reasonable
  • need to be able to have multiple API tokens - e.g. I have one token for us-west-2 and one for us-east-1
• have an ok experience for selecting expiration
  • can have tokens that never expire, but users must opt into them
• should not show up in help text
• should have a preview warning in case someone finds it