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/request_body.go

@@ -0,0 +1,69 @@
+package openapi3
+
+import (
+	"context"
+
+	"github.com/getkin/kin-openapi/jsoninfo"
+)
+
+// RequestBody is specified by OpenAPI/Swagger 3.0 standard.
+type RequestBody struct {
+	ExtensionProps
+	Description string  `json:"description,omitempty" yaml:"description,omitempty"`
+	Required    bool    `json:"required,omitempty" yaml:"required,omitempty"`
+	Content     Content `json:"content,omitempty" yaml:"content,omitempty"`
+}
+
+func NewRequestBody() *RequestBody {
+	return &RequestBody{}
+}
+
+func (requestBody *RequestBody) WithDescription(value string) *RequestBody {
+	requestBody.Description = value
+	return requestBody
+}
+
+func (requestBody *RequestBody) WithRequired(value bool) *RequestBody {
+	requestBody.Required = value
+	return requestBody
+}
+
+func (requestBody *RequestBody) WithContent(content Content) *RequestBody {
+	requestBody.Content = content
+	return requestBody
+}
+
+func (requestBody *RequestBody) WithJSONSchemaRef(value *SchemaRef) *RequestBody {
+	requestBody.Content = NewContentWithJSONSchemaRef(value)
+	return requestBody
+}
+
+func (requestBody *RequestBody) WithJSONSchema(value *Schema) *RequestBody {
+	requestBody.Content = NewContentWithJSONSchema(value)
+	return requestBody
+}
+
+func (requestBody *RequestBody) GetMediaType(mediaType string) *MediaType {
+	m := requestBody.Content
+	if m == nil {
+		return nil
+	}
+	return m[mediaType]
+}
+
+func (requestBody *RequestBody) MarshalJSON() ([]byte, error) {
+	return jsoninfo.MarshalStrictStruct(requestBody)
+}
+
+func (requestBody *RequestBody) UnmarshalJSON(data []byte) error {
+	return jsoninfo.UnmarshalStrictStruct(data, requestBody)
+}
+
+func (requestBody *RequestBody) Validate(c context.Context) error {
+	if v := requestBody.Content; v != nil {
+		if err := v.Validate(c); err != nil {
+			return err
+		}
+	}
+	return nil
+}