… `not_before`

In fido-authenticator, if we change the paths of RK  to be:
"rp_id.rk_id" instead of the current "rp_id/rk_id", we still want to be able
to iterate over the keys even though we only  know the "rp_id" and not the
"rk_id". Therefore we need to be able to stop at "rp_id.***" when giving "rp_id" in `not_before`

This is technically a breaking change because now, given the files:

- "aaa"
- "aaabbb"

 "read_dir_first"  with "aaa" as `not_before` would only yield "aaa"
due to littlefs-project/littlefs#923.
Now this will yield both files, and yield "aaabbb" first.

I beleive this behaviour is technically more correct as it is likely what
would be expected to be yield expecting alphabetical order
(though the order of the entries is still incorrect).