A Value contains a password or other secret value.
Use [methodValue.get] to get the actual secret data, such as a password. The secret data is not necessarily null-terminated,
unless the content type is "text/plain".
Each Value has a content type. For passwords, this is `text/plain`. Use [methodValue.get_content_type
] to look at the content type.
Value is reference counted and immutable. The secret data is only freed when all references have been released
via [methodValue.unref].