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/encoding.go

@@ -0,0 +1,93 @@
+package openapi3
+
+import (
+	"context"
+	"fmt"
+
+	"github.com/getkin/kin-openapi/jsoninfo"
+)
+
+// Encoding is specified by OpenAPI/Swagger 3.0 standard.
+type Encoding struct {
+	ExtensionProps
+
+	ContentType   string                `json:"contentType,omitempty" yaml:"contentType,omitempty"`
+	Headers       map[string]*HeaderRef `json:"headers,omitempty" yaml:"headers,omitempty"`
+	Style         string                `json:"style,omitempty" yaml:"style,omitempty"`
+	Explode       *bool                 `json:"explode,omitempty" yaml:"explode,omitempty"`
+	AllowReserved bool                  `json:"allowReserved,omitempty" yaml:"allowReserved,omitempty"`
+}
+
+func NewEncoding() *Encoding {
+	return &Encoding{}
+}
+
+func (encoding *Encoding) WithHeader(name string, header *Header) *Encoding {
+	return encoding.WithHeaderRef(name, &HeaderRef{
+		Value: header,
+	})
+}
+
+func (encoding *Encoding) WithHeaderRef(name string, ref *HeaderRef) *Encoding {
+	headers := encoding.Headers
+	if headers == nil {
+		headers = make(map[string]*HeaderRef)
+		encoding.Headers = headers
+	}
+	headers[name] = ref
+	return encoding
+}
+
+func (encoding *Encoding) MarshalJSON() ([]byte, error) {
+	return jsoninfo.MarshalStrictStruct(encoding)
+}
+
+func (encoding *Encoding) UnmarshalJSON(data []byte) error {
+	return jsoninfo.UnmarshalStrictStruct(data, encoding)
+}
+
+// SerializationMethod returns a serialization method of request body.
+// When serialization method is not defined the method returns the default serialization method.
+func (encoding *Encoding) SerializationMethod() *SerializationMethod {
+	sm := &SerializationMethod{Style: SerializationForm, Explode: true}
+	if encoding != nil {
+		if encoding.Style != "" {
+			sm.Style = encoding.Style
+		}
+		if encoding.Explode != nil {
+			sm.Explode = *encoding.Explode
+		}
+	}
+	return sm
+}
+
+func (encoding *Encoding) Validate(c context.Context) error {
+	if encoding == nil {
+		return nil
+	}
+	for k, v := range encoding.Headers {
+		if err := ValidateIdentifier(k); err != nil {
+			return nil
+		}
+		if err := v.Validate(c); err != nil {
+			return nil
+		}
+	}
+
+	// Validate a media types's serialization method.
+	sm := encoding.SerializationMethod()
+	switch {
+	case sm.Style == SerializationForm && sm.Explode,
+		sm.Style == SerializationForm && !sm.Explode,
+		sm.Style == SerializationSpaceDelimited && sm.Explode,
+		sm.Style == SerializationSpaceDelimited && !sm.Explode,
+		sm.Style == SerializationPipeDelimited && sm.Explode,
+		sm.Style == SerializationPipeDelimited && !sm.Explode,
+		sm.Style == SerializationDeepObject && sm.Explode:
+		// it is a valid
+	default:
+		return fmt.Errorf("Serialization method with style=%q and explode=%v is not supported by media type", sm.Style, sm.Explode)
+	}
+
+	return nil
+}