Add a readonly backend

[donny-dont]

Create a backend which provides a read only view over another backend. This could be used to prevent a user from modifying a backend.

ConfigMap is expanded to support for nesting of configs.

[donny-dont]

With the ability to specify nested configs this can be expanded to caching. So as an example if a file is present locally that one would be used but if its not then it would hit a cloud storage service cache it locally and return the file to the caller. This is similar to what's possible in https://github.com/spf13/afero.

I have that in a separate config that's dependent on having the nested configs. Let me know if this or the caching implementation is of interest to you all.