APPS: Add OSSL_STORE loader for engine keys

APPS: Add OSSL_STORE loader for engine keys

The idea is to be able to have our apps load engine keys using a URI:

    org.openssl.engine:{engineid}:{keyid}

This is legacy, but added for the time being to support keys given to
the application like this:

    -engine {engineid} -key {keyid} -keyform ENGINE

This latter form is recognised internally, and rewritten into the URI
form.

Reviewed-by: David von Oheimb <david.von.oheimb@siemens.com>
(Merged from https://github.com/openssl/openssl/pull/13570)