go: vendor the oapi-codegen cmd

See the comment in tools.go, I cannot fully explain what's happening here.
Somehow, Go 1.14 wants to use the vendored version of oapi-codegen but
without this file, oapi-codegen isn't vendored so the generation fails.

Signed-off-by: Ondřej Budai <ondrej@budai.cz>

vendor/github.com/deepmap/oapi-codegen/pkg/codegen/extension.go

@@ -0,0 +1,25 @@
+package codegen
+
+import (
+	"encoding/json"
+	"fmt"
+
+	"github.com/pkg/errors"
+)
+
+const (
+	extPropGoType = "x-go-type"
+)
+
+func extTypeName(extPropValue interface{}) (string, error) {
+	raw, ok := extPropValue.(json.RawMessage)
+	if !ok {
+		return "", fmt.Errorf("failed to convert type: %T", extPropValue)
+	}
+	var name string
+	if err := json.Unmarshal(raw, &name); err != nil {
+		return "", errors.Wrap(err, "failed to unmarshal json")
+	}
+
+	return name, nil
+}