PROV & STORE: Don't decode keys in the 'file:' store loader

PROV & STORE: Don't decode keys in the 'file:' store loader

This makes the 'file:' store loader only read the file, and only decode
down to a base level binary format, and simply pass that blob of data
back to the OSSL_FUNC_store_load() object callback.

This offloads the decoding into specific OpenSSL types to libcrypto,
which takes away the issue of origins, which provider is it that holds
the key (or other future types of objects).

Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/15981)