This commit adds a new option --check-key to endpoint health command
for health checking with a custom key. It is mainly for avoiding
permission problem.
This commit adds a functionality of attaching an auth token to gRPC
connection as a per RPC credential.
For doing this, this commit lets clientv3.Client.Dial() create a
dedicated gRPC connection for doing authentication. With the dedicated
connection, the client calls Authenticate() RPC and obtain its
token. The token is attached to the main gRPC connection with
grpc.WithPerRPCCredentials().
This commit also adds a new option --username to etcdctl (v3). With
this option, etcdctl attaches its auth token to the main gRPC
connection (currently it is not used at all).
