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/getkin/kin-openapi/openapi3/serialization_method.go

@@ -0,0 +1,17 @@
+package openapi3
+
+const (
+	SerializationSimple         = "simple"
+	SerializationLabel          = "label"
+	SerializationMatrix         = "matrix"
+	SerializationForm           = "form"
+	SerializationSpaceDelimited = "spaceDelimited"
+	SerializationPipeDelimited  = "pipeDelimited"
+	SerializationDeepObject     = "deepObject"
+)
+
+// SerializationMethod describes a serialization method of HTTP request's parameters and body.
+type SerializationMethod struct {
+	Style   string
+	Explode bool
+}