PXD-1350 ⁃ additional logic for iam object GooglePolicyMember

[philloooo]

add an is_user() to GooglePolicyMember

add an is_service_account() to GooglePolicyMember

add an is_group() to GooglePolicyMember

additionally, logic for GooglePolicyMember for specific service account types (currently have an enumeration for these in cirrus:

is_compute_engine_default_service_account()

is_google_api_service_account()

is_compute_engine_api_service_account()

is_user_managed_service_account()

Bonus points for updating fence to use these new functions instead of importing the COMPUTE_ENGINE_DEFAULT_SERVICE_ACCOUNT types and checking against them