[Golang] Move utility functions from client.mustache to utils.mustache (#19001)

* [Golang] Move utility functions from client.mustache to utils.mustache

* re-generate Go samples

* re-generate Go samples again

* re-generate samples once more

* update go samples

---------

Co-authored-by: Martin Lakov <martin.lakov@ocado.com>

samples/client/echo_api/go/client.go

@@ -519,18 +519,6 @@ func addFile(w *multipart.Writer, fieldName, path string) error {
 	return err
 }
 
-// Prevent trying to import "fmt"
-func reportError(format string, a ...interface{}) error {
-	return fmt.Errorf(format, a...)
-}
-
-// A wrapper for strict JSON decoding
-func newStrictDecoder(data []byte) *json.Decoder {
-	dec := json.NewDecoder(bytes.NewBuffer(data))
-	dec.DisallowUnknownFields()
-	return dec
-}
-
 // Set request body from an interface{}
 func setBody(body interface{}, contentType string) (bodyBuf *bytes.Buffer, err error) {
 	if bodyBuf == nil {

samples/client/echo_api/go/utils.go

@@ -12,7 +12,9 @@ Contact: team@openapitools.org
 package openapi
 
 import (
+	"bytes"
 	"encoding/json"
+	"fmt"
 	"reflect"
 	"time"
 )
@@ -346,3 +348,15 @@ func IsNil(i interface{}) bool {
 type MappedNullable interface {
 	ToMap() (map[string]interface{}, error)
 }
+
+// A wrapper for strict JSON decoding
+func newStrictDecoder(data []byte) *json.Decoder {
+	dec := json.NewDecoder(bytes.NewBuffer(data))
+	dec.DisallowUnknownFields()
+	return dec
+}
+
+// Prevent trying to import "fmt"
+func reportError(format string, a ...interface{}) error {
+	return fmt.Errorf(format, a...)
+}