https://issues.redhat.com/browse/RUN-3647 added a "no-op" GRPC endpoint to the podman system service socket, and set up some patterns for how additional endpoints could be added to it going forward.

We want to be able to offer build-related RPCs from buildah as well, with the idea that podman will be able to add the same endpoints to its service, but unlike podman, buildah provides no listening socket for us to add a no-op service to.

We'll add an experimental "buildah rpc" command which, like "buildah unshare", will run the specified command with the location of the listening socket provided in the environment, and add a flag to the "buildah unshare" command to have it provide the same endpoint to a command being run under it.