add --platform argument to most commands

[cyphar]

We should probably just support the github.com/containerd/platforms format for defining platform specifications. We need quite a few commands to support this:

• umoci config should be able to set the platform for the config blob. It might also make sense for us to modify the descriptor-embedded platform information to match...
• umoci unpack should be able to
• umoci new probably needs this as a config option, at least to be able to configure the descriptor platform annotation (and the configuration too).
  • This is somewhat linked to #637, and umoci manifest would need to support setting this too.