Add admin get/set config keys API (#6113)

This PR adds two new admin APIs in Minio server and madmin package:
- GetConfigKeys(keys []string) ([]byte, error)
- SetConfigKeys(params map[string]string) (err error)

A key is a path in Minio configuration file, (e.g. notify.webhook.1)

The user will always send a string value when setting it in the config file,
the API will know how to convert the value to the appropriate type. The user
is also able to set a raw json.

Before setting a new config, Minio will validate all fields and try to connect
to notification targets if available.

cmd/test-utils_test.go

@@ -502,7 +502,7 @@ func newTestConfig(bucketLocation string, obj ObjectLayer) (err error) {
 	globalServerConfig.SetRegion(bucketLocation)
 
 	// Save config.
-	return saveServerConfig(obj, globalServerConfig)
+	return saveServerConfig(context.Background(), obj, globalServerConfig)
 }
 
 // Deleting the temporary backend and stopping the server.