b/147430272: Add delegates option for the future implementation of IamCredentialsAPI
b/147430272: Add delegates option for the future implementation of IamCredentialsAPI

Change-Id: Ia9487ebf5e59cdf744dab5884856f75c9296e4a6

GoogleCloudPlatform/esp-v2,GoogleCloudPlatform/esp-v2,GoogleCloudPlatform/esp-v2,GoogleCloudPlatform/esp-v2,GoogleCloudPlatform/esp-v2