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/examples.go

@@ -0,0 +1,29 @@
+package openapi3
+
+import (
+	"github.com/getkin/kin-openapi/jsoninfo"
+)
+
+// Example is specified by OpenAPI/Swagger 3.0 standard.
+type Example struct {
+	ExtensionProps
+
+	Summary       string      `json:"summary,omitempty" yaml:"summary,omitempty"`
+	Description   string      `json:"description,omitempty" yaml:"description,omitempty"`
+	Value         interface{} `json:"value,omitempty" yaml:"value,omitempty"`
+	ExternalValue string      `json:"externalValue,omitempty" yaml:"externalValue,omitempty"`
+}
+
+func NewExample(value interface{}) *Example {
+	return &Example{
+		Value: value,
+	}
+}
+
+func (example *Example) MarshalJSON() ([]byte, error) {
+	return jsoninfo.MarshalStrictStruct(example)
+}
+
+func (example *Example) UnmarshalJSON(data []byte) error {
+	return jsoninfo.UnmarshalStrictStruct(data, example)
+}