As we're trying to move away from gob for serialising data over the
wire, this commit adds the capability for Packer to pick dynamically
between gob or protobuf for the serialisation format to communicate with
plugins.
As it stands, if all the plugins discovered are compatible with
protobuf, and we have not forced gob usage, protobuf will be the
serialisation format picked.
If any plugin is not compatible with protobuf, gob will be used for
communicating with all the plugins that will be used over the course of
a command.
9f6f0ba6a2