STORE 'file' scheme loader: add support for containers

Containers are objects that are containers for a bunch of other
objects with types we recognise but aren't readable in a stream.  Such
containers are read and parsed, and their content is cached, to be
served one object at a time.

This extends the FILE_HANDLER type to include a function to destroy
the cache and a function to simulate the EOF check.

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