Allow parameters using a data format to be unmarshaled

[Jthomas54]

It can be convenient to store a parameter value as json or some other data format. It would be nice to be able to express this so common formats can be unmarshaled or custom unmarshaling can occur.

For example, the below struct would load the parameter and then unmarshal the value using the json decoder, applying rules of the Metadata structs json tags.

type Cfg struct {
  Metadata Metadata `ssm:"/app/metadata, json"`
}