Allow full management of IAM parent fields

[melinath]

What kind of contribution is this issue about?

MMv1-based resource

Details

IAM resources can have multiple parent fields, but it is difficult to modify their behavior. We only allow overriding parent_resource_attribute and setting a custom diff suppress for the final parent field. We don't allow (for example) adding validation for a parent field, which could be necessary to have parity with the parent resource, or allow a custom diff suppress for fields other than the final resource.

We should restructure the iam_policy configuration to allow easier customization of all parent fields, similar to resource fields.

References

Ran into this while reviewing GoogleCloudPlatform/magic-modules#10786, which is forced to use a diff suppress to replicate missing validation on one parent field, and is also missing a diff suppress that is present for IAM policies for the parent resource. (Unclear if this will cause problems.)