Server integration test for Device Flow (#3)

Extracted test cases from OAuth2Code flow tests to reuse in device flow

deviceHandler unit tests to test specific device endpoints

Include client secret as an optional parameter for standards compliance

Signed-off-by: justin-slowik <justin.slowik@thermofisher.com>

storage/etcd/etcd_test.go

@@ -44,6 +44,8 @@ func cleanDB(c *conn) error {
 		passwordPrefix,
 		offlineSessionPrefix,
 		connectorPrefix,
+		deviceRequestPrefix,
+		deviceTokenPrefix,
 	} {
 		_, err := c.db.Delete(ctx, prefix, clientv3.WithPrefix())
 		if err != nil {