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/content.go

@@ -0,0 +1,77 @@
+package openapi3
+
+import (
+	"context"
+	"strings"
+)
+
+// Content is specified by OpenAPI/Swagger 3.0 standard.
+type Content map[string]*MediaType
+
+func NewContent() Content {
+	return make(map[string]*MediaType, 4)
+}
+
+func NewContentWithJSONSchema(schema *Schema) Content {
+	return Content{
+		"application/json": NewMediaType().WithSchema(schema),
+	}
+}
+func NewContentWithJSONSchemaRef(schema *SchemaRef) Content {
+	return Content{
+		"application/json": NewMediaType().WithSchemaRef(schema),
+	}
+}
+
+func (content Content) Get(mime string) *MediaType {
+	// If the mime is empty then short-circuit to the wildcard.
+	// We do this here so that we catch only the specific case of
+	// and empty mime rather than a present, but invalid, mime type.
+	if mime == "" {
+		return content["*/*"]
+	}
+	// Start by making the most specific match possible
+	// by using the mime type in full.
+	if v := content[mime]; v != nil {
+		return v
+	}
+	// If an exact match is not found then we strip all
+	// metadata from the mime type and only use the x/y
+	// portion.
+	i := strings.IndexByte(mime, ';')
+	if i < 0 {
+		// If there is no metadata then preserve the full mime type
+		// string for later wildcard searches.
+		i = len(mime)
+	}
+	mime = mime[:i]
+	if v := content[mime]; v != nil {
+		return v
+	}
+	// If the x/y pattern has no specific match then we
+	// try the x/* pattern.
+	i = strings.IndexByte(mime, '/')
+	if i < 0 {
+		// In the case that the given mime type is not valid because it is
+		// missing the subtype we return nil so that this does not accidentally
+		// resolve with the wildcard.
+		return nil
+	}
+	mime = mime[:i] + "/*"
+	if v := content[mime]; v != nil {
+		return v
+	}
+	// Finally, the most generic match of */* is returned
+	// as a catch-all.
+	return content["*/*"]
+}
+
+func (content Content) Validate(c context.Context) error {
+	for _, v := range content {
+		// Validate MediaType
+		if err := v.Validate(c); err != nil {
+			return err
+		}
+	}
+	return nil
+}