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/external_docs.go

@@ -0,0 +1,21 @@
+package openapi3
+
+import (
+	"github.com/getkin/kin-openapi/jsoninfo"
+)
+
+// ExternalDocs is specified by OpenAPI/Swagger standard version 3.0.
+type ExternalDocs struct {
+	ExtensionProps
+
+	Description string `json:"description,omitempty"`
+	URL         string `json:"url,omitempty"`
+}
+
+func (e *ExternalDocs) MarshalJSON() ([]byte, error) {
+	return jsoninfo.MarshalStrictStruct(e)
+}
+
+func (e *ExternalDocs) UnmarshalJSON(data []byte) error {
+	return jsoninfo.UnmarshalStrictStruct(data, e)
+}