The hcp-sbom provisioner is a provisioner that acts essentially like a
download-only file provisioner, which also verifies the file downloaded
is a SPDX/CycloneDX JSON-encoded SBOM file, and sets up its upload to
HCP Packer later on.
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.
The plugin and plugins command had a name that was close, and while
plugin is not supposed to be directly called by Packer users, this could
happen by accident while trying to execute packer plugins subcommands,
and when it does, the error messages are far from explicit, so unless
they understand what Packer is doing here, they'll likely be lost.
To reduce the risk of confusion, we rename the command to run packer
embedded components as execute.
