Add dial-stdio command

This allows the buildx CLI to act a proxy to the configured instance.
It allows external code to use buildx itself as a driver for connecting
to buildkitd instances.

Instance and node selection should follow the same semantics as as
`buildx build`, including taking into account the `BUILDX_BUILDER` env
var and the `--builder` global flag.

Signed-off-by: Brian Goff <cpuguy83@gmail.com>

tests/integration_test.go

@@ -29,6 +29,7 @@ func TestIntegration(t *testing.T) {
 	tests = append(tests, versionTests...)
 	tests = append(tests, createTests...)
 	tests = append(tests, rmTests...)
+	tests = append(tests, dialstdioTests...)
 	testIntegration(t, tests...)
 }