Add WorkloadIdentityFederation#identity_pool_resource_name

What does this MR do and why?

This MR adds the new identity_pool_resource_name method to Integrations::GoogleCloudPlatform::WorkloadIdentityFederation integration that'll be used for IAM policies. The related thread

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

How to set up and validate locally

1. Simulate a SaaS instance in the local GDK

2. Execute the following in the Rails console:

Feature.enable(:google_cloud_workload_identity_federation)
i = Integrations::GoogleCloudPlatform::WorkloadIdentityFederation.create(workload_identity_federation_project_id: 'project_id', workload_identity_federation_project_number: '123456789', workload_identity_pool_id: 'pool_id', workload_identity_pool_provider_id: 'provider_id', project: Project.last, type: 'Integrations::GoogleCloudPlatform::WorkloadIdentityFederation', active: true)
i.identity_pool_resource_name # verify the value

Related to #442032