Support for nested folders

[tmpfs]

This has been requested and we need to consider how to elegantly support this in the context of the current design.

When initially designing the sdk I thought nested folders was overkill and tagging was a better solution but I can see why some people prefer the nested folder paradigm.

[tmpfs]

We could create a Directory secret variant that could use the nested secrets on UserData to build a tree of secrets that would represent a virtual directory structure.

It would be quite straightforward to implement but would come with the caveat that UserData for directories would not be usable, I don't know if custom user data for nested folders would be useful or not.

Alternatively we could just add a children field so as not to overload UserData, for example:

enum Secret {
    #[serde(rename_all = "camelCase")]
    Directory {
        /// Nested secrets and directories.
        children: Vec<Secret>,
        /// Custom user data.
        #[serde(default, skip_serializing_if = "UserData::is_default")]
        user_data: UserData,
    },
}