With the next minor version of the SDK, we'll introduce experimental
protobuf support for serialising data between Packer and plugins.
This is exposed to plugins under the `PACKER_PLUGIN_PB` environment
variable, which is automatically set when all the plugins able to be
loaded by Packer (i.e. the highest version compatible with Packer's
loading process) are capable to communicate with Protobuf.
If any plugin uses version 5.0 of the API, we default to using Gob,
thereby maintaining retro-compatibility will all the existing plugins.
Lucas Bajolet
bda59b4d0a