Support pkcs8 from Apple APNs

[Otto]

Hi,
After spending a few hours on trying to import Apple Mapkit p8 file, which is pkcs8, i have stumbled on someones post here:
dgrijalva/jwt-go#179

It includes a working function to import the private key, i've tested it and it works.
While i am not the author of the post in the link, i feel it would be beneficial to many people to have this function be included in your repo.
Would you be interested in a pull request with the above mentioned change ?
I will make sure to include the original author.

[Christian Banse]

Isn't this solved by this function here?

jwt/ecdsa_utils.go

Lines 15 to 40 in b88a60f

 // ParseECPrivateKeyFromPEM parses a PEM encoded Elliptic Curve Private Key Structure 
 func ParseECPrivateKeyFromPEM(key []byte) (*ecdsa.PrivateKey, error) { 
 	var err error 
  
 	// Parse PEM block 
 	var block *pem.Block 
 	if block, _ = pem.Decode(key); block == nil { 
 		return nil, ErrKeyMustBePEMEncoded 
 	} 
  
 	// Parse the key 
 	var parsedKey interface{} 
 	if parsedKey, err = x509.ParseECPrivateKey(block.Bytes); err != nil { 
 		if parsedKey, err = x509.ParsePKCS8PrivateKey(block.Bytes); err != nil { 
 			return nil, err 
 		} 
 	} 
  
 	var pkey *ecdsa.PrivateKey 
 	var ok bool 
 	if pkey, ok = parsedKey.(*ecdsa.PrivateKey); !ok { 
 		return nil, ErrNotECPrivateKey 
 	} 
  
 	return pkey, nil 
 }