Add API to list all available signing methods

Migrated from https://github.com/dgrijalva/jwt-go/issues/410:

>  trygvis commented on Jun 17, 2020
> 
> I'm building a CLI to create tokens, and it would be nice if there was a way to find all registered signing methods.
> 
> Something like
> 

`func GetSigningMethods() ([]SigningMethod)`