Attempt to dry up CLI client, add proepr config

This commit is trying to DRY up the initiation of the gRPC client in
each command:

It renames the function to CLI instead of GRPC as it actually set up a
CLI client, not a generic grpc client

It also moves the configuration of address, timeout (which is now
consistent) and api to use Viper, allowing users to set it via env vars
and configuration file

app.go

@@ -86,6 +86,8 @@ type Config struct {
 
 	OIDC OIDCConfig
 
+	CLI CLIConfig
+
 	MaxMachineRegistrationDuration     time.Duration
 	DefaultMachineRegistrationDuration time.Duration
 }
@@ -104,6 +106,13 @@ type DERPConfig struct {
 	UpdateFrequency time.Duration
 }
 
+type CLIConfig struct {
+	Address  string
+	APIKey   string
+	Insecure bool
+	Timeout  time.Duration
+}
+
 // Headscale represents the base app of the service.
 type Headscale struct {
 	cfg        Config