PEM: Make PEM_write_bio_PrivateKey_traditional() handle provider-native keys

PEM: Make PEM_write_bio_PrivateKey_traditional() handle provider-native keys

PEM_write_bio_PrivateKey_traditional() didn't handle provider-native
keys very well.  Originally, it would simply use the corresponding
encoder, which is likely to output modern PEM (not "traditional").

PEM_write_bio_PrivateKey_traditional() is now changed to try and get a
legacy copy of the input EVP_PKEY, and use that copy for traditional
output, if it has such support.

Internally, evp_pkey_copy_downgraded() is added, to be used when
evp_pkey_downgrade() is too intrusive for what it's needed for.

Reviewed-by: Shane Lontis <shane.lontis@oracle.com>
(Merged from https://github.com/openssl/openssl/pull/12738)