KEYMGMT: Add key loading function OSSL_FUNC_keymgmt_load()

KEYMGMT: Add key loading function OSSL_FUNC_keymgmt_load()

This function is used to create a keydata for a key that libcrypto
only has a reference to.

This introduces provider references, the contents which only the
provider know how to interpret.  Outside of the provider, this is just
an array of bytes.

Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Shane Lontis <shane.lontis@oracle.com>
(Merged from https://github.com/openssl/openssl/pull/12410)