Feature: Make bucket object lock default retention configurable

[HannesHerrmannAA]

Problem description

Configuring default retention of stackit buckets is not configurable via terraform, but can be set via the portal or via the API. Compare https://docs.stackit.cloud/products/storage/object-storage/how-tos/object-lock-default-retention/

This should be added to the terraform provider, so all api based configs can be set.

Proposed solution

Add a terraform config which allows to set the default retention policy:

Something like:

resource stackit_objectstorage_bucket_default_retention "example" {
  bucket = ...
  project_id = ...  

   mode = "COMPLIANCE" 
   retention_days = 14
}