Support AWS Signature V2 for Mattermost for S3 storage. (#6462)

Certain S3 compatible servers only use Legacy Signature (AWS
Signature V2), current code only supports signature v4.

This PR adds facility to click a button on the UI to enable
legacy signature with S3 compatible servers.

api4/apitestlib.go

@@ -626,13 +626,21 @@ func readTestFile(name string) ([]byte, error) {
 	}
 }
 
+func s3New(endpoint, accessKey, secretKey string, secure bool, signV2 bool) (*s3.Client, error) {
+	if signV2 {
+		return s3.NewV2(endpoint, accessKey, secretKey, secure)
+	}
+	return s3.NewV4(endpoint, accessKey, secretKey, secure)
+}
+
 func cleanupTestFile(info *model.FileInfo) error {
 	if utils.Cfg.FileSettings.DriverName == model.IMAGE_DRIVER_S3 {
 		endpoint := utils.Cfg.FileSettings.AmazonS3Endpoint
 		accessKey := utils.Cfg.FileSettings.AmazonS3AccessKeyId
 		secretKey := utils.Cfg.FileSettings.AmazonS3SecretAccessKey
 		secure := *utils.Cfg.FileSettings.AmazonS3SSL
-		s3Clnt, err := s3.New(endpoint, accessKey, secretKey, secure)
+		signV2 := *utils.Cfg.FileSettings.AmazonS3SignV2
+		s3Clnt, err := s3New(endpoint, accessKey, secretKey, secure, signV2)
 		if err != nil {
 			return err
 		}