This is just a note to future maintainers to clarify the meaning of the
initial end-user-facing error message that says this interface is only
for Terraform Cloud.
Targeting Terraform Cloud as a client first is just a means to get
practical experience with this new style of integration before committing
to support it as a general API. We would still like to make this a
general-purpose integration point with compatibility guarantees in a later
release, once this approach is more proven.
This doesn't actually do anything useful yet, but this does at least make
the RPC server accessible to external callers where they can handshake and
then get an error saying that nothing else is implemented.
