FR: Add workload identity federation for service accounts

[hinricht]

Problem description

I'd like to configure Workload identity federation for service accounts with the STACKIT Terraform provider, so I have the config in git, instead of manually configuring it through the STACKIT portal (as socumented in https://docs.stackit.cloud/platform/access-and-identity/service-accounts/how-tos/manage-service-account-federations/#create-a-federated-identity-provider).

Proposed solution

Add the neccessary resources to configure Workload identity federation for service accounts.

Alternative solutions (optional)

• Manually configuring it through the STACKIT portal (as socumented in https://docs.stackit.cloud/platform/access-and-identity/service-accounts/how-tos/manage-service-account-federations/#create-a-federated-identity-provider).

[JorTurFer]

This is in progress #1314