Decoding PKCS#8: separate decoding of encrypted and unencrypted PKCS#8

Decoding PKCS#8: separate decoding of encrypted and unencrypted PKCS#8

This has us switch from the 'structure' "pkcs8" to "PrivateKeyInfo",
which is sensible considering we already have "SubjectPublicKeyInfo".
We also add "EncryptedPrivateKeyInfo", and use it for a special decoder
that detects and decrypts an EncryptedPrivateKeyInfo structured DER
blob into a PrivateKeyInfo structured DER blob and passes that on to
the next decoder implementation.

The result of this change is that PKCS#8 decryption should only happen
once per decoding instead of once for every expected key type.
Furthermore, this new decoder implementation sets the data type to the
OID of the algorithmIdentifier field, thus reducing how many decoder
implementations are tentativaly run further down the call chain.

Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/15498)