STORE 'file' scheme loader: Add directory listing capability

STORE 'file' scheme loader: Add directory listing capability

This has it recognised when the given path is a directory.  In that
case, the file loader will give back a series of names, all as URI
formatted as possible given the incoming URI.

Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/3542)