Support pkcs8 from Apple APNs

[conotto]

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

[oxisto]

Isn't this solved by this function here?

https://github.com/golang-jwt/jwt/blob/b88a60f2d771b225195859f27dd5ae1901ffb483/ecdsa_utils.go#L15-L40