Add built-in credential provider for tokens
Add built-in credential provider for tokens

This was previously part of the very long command/auth.go file, where it
mimmicked the same API as other handlers. By making it a builtin
credential, we can remove a lot of conditional logic for token-based
authentication.

joelthompson/vault,rnaveiras/vault,hartsock/vault,joelthompson/vault,hartsock/vault,hashicorp/vault,naunga/vault,rnaveiras/vault,rnaveiras/vault,Aloomaio/vault,mgk/vault,Aloomaio/vault,joelthompson/vault,joelthompson/vault,quixoten/vault,hashicorp/vault,kingland/vault,mgk/vault,mgk/vault,kingland/vault,hashicorp/vault,mgk/vault,hartsock/vault,hashicorp/vault,kingland/vault,hashicorp/vault,Aloomaio/vault,naunga/vault,hartsock/vault,kingland/vault,rnaveiras/vault,quixoten/vault,hashicorp/vault,hartsock/vault,rnaveiras/vault,quixoten/vault,quixoten/vault,kingland/vault,Aloomaio/vault