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/deepmap/oapi-codegen/pkg/codegen/inline.go

@@ -0,0 +1,77 @@
+// Copyright 2019 DeepMap, Inc.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+package codegen
+
+import (
+	"bufio"
+	"bytes"
+	"compress/gzip"
+	"encoding/base64"
+	"fmt"
+	"text/template"
+
+	"github.com/getkin/kin-openapi/openapi3"
+)
+
+// This generates a gzipped, base64 encoded JSON representation of the
+// swagger definition, which we embed inside the generated code.
+func GenerateInlinedSpec(t *template.Template, swagger *openapi3.Swagger) (string, error) {
+	// Marshal to json
+	encoded, err := swagger.MarshalJSON()
+	if err != nil {
+		return "", fmt.Errorf("error marshaling swagger: %s", err)
+	}
+
+	// gzip
+	var buf bytes.Buffer
+	zw, err := gzip.NewWriterLevel(&buf, gzip.BestCompression)
+	if err != nil {
+		return "", fmt.Errorf("error creating gzip compressor: %s", err)
+	}
+	_, err = zw.Write(encoded)
+	if err != nil {
+		return "", fmt.Errorf("error gzipping swagger file: %s", err)
+	}
+	err = zw.Close()
+	if err != nil {
+		return "", fmt.Errorf("error gzipping swagger file: %s", err)
+	}
+	str := base64.StdEncoding.EncodeToString(buf.Bytes())
+
+	var parts []string
+	const width = 80
+
+	// Chop up the string into an array of strings.
+	for len(str) > width {
+		part := str[0:width]
+		parts = append(parts, part)
+		str = str[width:]
+	}
+	if len(str) > 0 {
+		parts = append(parts, str)
+	}
+
+	// Generate inline code.
+	buf.Reset()
+	w := bufio.NewWriter(&buf)
+	err = t.ExecuteTemplate(w, "inline.tmpl", parts)
+	if err != nil {
+		return "", fmt.Errorf("error generating inlined spec: %s", err)
+	}
+	err = w.Flush()
+	if err != nil {
+		return "", fmt.Errorf("error flushing output buffer for inlined spec: %s", err)
+	}
+	return buf.String(), nil
+}