versioning: plugins report a list of supported versions

Further progress on versioning support (Issue #266).
Bump CNI spec version to 0.3.0

invoke/exec.go

@@ -23,6 +23,7 @@ import (
 	"os/exec"
 
 	"github.com/containernetworking/cni/pkg/types"
+	"github.com/containernetworking/cni/pkg/version"
 )
 
 func pluginErr(err error, output []byte) error {
@@ -57,6 +58,15 @@ func ExecPluginWithoutResult(pluginPath string, netconf []byte, args CNIArgs) er
 	return err
 }
 
+func ExecPluginForVersion(pluginPath string) (version.PluginInfo, error) {
+	stdoutBytes, err := execPlugin(pluginPath, nil, &Args{Command: "VERSION"})
+	if err != nil {
+		return nil, err
+	}
+
+	return version.Decode(stdoutBytes)
+}
+
 func execPlugin(pluginPath string, netconf []byte, args CNIArgs) ([]byte, error) {
 	return defaultRawExec.ExecPlugin(pluginPath, netconf, args.AsEnv())
 }