chore: cleanup unneeded device storage methods (#399)

BREAKING CHANGE, removes methods from DeviceAuthorizationStorage:

- GetDeviceAuthorizationByUserCode
- CompleteDeviceAuthorization
- DenyDeviceAuthorization

The methods are now moved to examples as something similar can be
userful for implementers.

example/server/exampleop/device.go

@@ -1,6 +1,7 @@
 package exampleop
 
 import (
+	"context"
 	"errors"
 	"fmt"
 	"io"
@@ -16,6 +17,18 @@ import (
 type deviceAuthenticate interface {
 	CheckUsernamePasswordSimple(username, password string) error
 	op.DeviceAuthorizationStorage
+
+	// GetDeviceAuthorizationByUserCode resturns the current state of the device authorization flow,
+	// identified by the user code.
+	GetDeviceAuthorizationByUserCode(ctx context.Context, userCode string) (*op.DeviceAuthorizationState, error)
+
+	// CompleteDeviceAuthorization marks a device authorization entry as Completed,
+	// identified by userCode. The Subject is added to the state, so that
+	// GetDeviceAuthorizatonState can use it to create a new Access Token.
+	CompleteDeviceAuthorization(ctx context.Context, userCode, subject string) error
+
+	// DenyDeviceAuthorization marks a device authorization entry as Denied.
+	DenyDeviceAuthorization(ctx context.Context, userCode string) error
 }
 
 type deviceLogin struct {