feat: allow anonymous access for s3proxy backend

* Update client.go to support anonymous S3 access

* Update s3.go to make access and secret parameters optional

* Update example.conf for more clear S3 access and secret usage

Fixes #1836

cmd/versitygw/s3.go

@@ -45,7 +45,6 @@ to an s3 storage backend service.`,
 				Name:        "access",
 				Usage:       "s3 proxy server access key id",
 				Value:       "",
-				Required:    true,
 				EnvVars:     []string{"VGW_S3_ACCESS_KEY"},
 				Destination: &s3proxyAccess,
 				Aliases:     []string{"a"},
@@ -54,7 +53,6 @@ to an s3 storage backend service.`,
 				Name:        "secret",
 				Usage:       "s3 proxy server secret access key",
 				Value:       "",
-				Required:    true,
 				EnvVars:     []string{"VGW_S3_SECRET_KEY"},
 				Destination: &s3proxySecret,
 				Aliases:     []string{"s"},