Add evp_keymgmt_clear_pkey_cache() and use it

Add evp_keymgmt_clear_pkey_cache() and use it

This function clears the cache of provider key references, and is used
in evp_keymgmt_export_to_provider() when the internal key is dirty, as
well as by EVP_PKEY_free_it().

Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/9312)