Add OAuth2 / JWT token authentication

Changelog:

• Add OAuth2 / JWT token authentication #648 #649

You can now use --token or -k to pass a JWT token through as a Bearer token for gateways implementing OAuth2/OIDC/JWT-validation.

To start the flow use faas-cli auth --help and then once you have token pass it to any command such as faas-cli list.

Update today: https://docs.openfaas.com/cli/install/