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/response.go

@@ -0,0 +1,92 @@
+package openapi3
+
+import (
+	"context"
+	"errors"
+	"strconv"
+
+	"github.com/getkin/kin-openapi/jsoninfo"
+)
+
+// Responses is specified by OpenAPI/Swagger 3.0 standard.
+type Responses map[string]*ResponseRef
+
+func NewResponses() Responses {
+	r := make(Responses)
+	r["default"] = &ResponseRef{Value: NewResponse().WithDescription("")}
+	return r
+}
+
+func (responses Responses) Default() *ResponseRef {
+	return responses["default"]
+}
+
+func (responses Responses) Get(status int) *ResponseRef {
+	return responses[strconv.FormatInt(int64(status), 10)]
+}
+
+func (responses Responses) Validate(c context.Context) error {
+	if len(responses) == 0 {
+		return errors.New("the responses object MUST contain at least one response code")
+	}
+	for _, v := range responses {
+		if err := v.Validate(c); err != nil {
+			return err
+		}
+	}
+	return nil
+}
+
+// Response is specified by OpenAPI/Swagger 3.0 standard.
+type Response struct {
+	ExtensionProps
+	Description *string               `json:"description,omitempty" yaml:"description,omitempty"`
+	Headers     map[string]*HeaderRef `json:"headers,omitempty" yaml:"headers,omitempty"`
+	Content     Content               `json:"content,omitempty" yaml:"content,omitempty"`
+	Links       map[string]*LinkRef   `json:"links,omitempty" yaml:"links,omitempty"`
+}
+
+func NewResponse() *Response {
+	return &Response{}
+}
+
+func (response *Response) WithDescription(value string) *Response {
+	response.Description = &value
+	return response
+}
+
+func (response *Response) WithContent(content Content) *Response {
+	response.Content = content
+	return response
+}
+
+func (response *Response) WithJSONSchema(schema *Schema) *Response {
+	response.Content = NewContentWithJSONSchema(schema)
+	return response
+}
+
+func (response *Response) WithJSONSchemaRef(schema *SchemaRef) *Response {
+	response.Content = NewContentWithJSONSchemaRef(schema)
+	return response
+}
+
+func (response *Response) MarshalJSON() ([]byte, error) {
+	return jsoninfo.MarshalStrictStruct(response)
+}
+
+func (response *Response) UnmarshalJSON(data []byte) error {
+	return jsoninfo.UnmarshalStrictStruct(data, response)
+}
+
+func (response *Response) Validate(c context.Context) error {
+	if response.Description == nil {
+		return errors.New("a short description of the response is required")
+	}
+
+	if content := response.Content; content != nil {
+		if err := content.Validate(c); err != nil {
+			return err
+		}
+	}
+	return nil
+}