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/typed-errors.go

@@ -50,6 +50,9 @@ var errRPCAPIVersionUnsupported = errors.New("Unsupported rpc API version")
 // errServerTimeMismatch - server times are too far apart.
 var errServerTimeMismatch = errors.New("Server times are too far apart")
 
+// errOperationTimedOut
+var errOperationTimedOut = errors.New("Operation timed out")
+
 // errInvalidBucketName - bucket name is reserved for Minio, usually
 // returned for 'minio', '.minio.sys', buckets with capital letters.
 var errInvalidBucketName = errors.New("The specified bucket is not valid")