Object storage misc fixes (#82)

* Fix wrong reference

* Fix schema

* Fix mapFields not fetching credentials group id

* Change expiration timestamp

* Fix schema

* Remove fields that don't come in the GET response

* Add RFC3339SecondsOnly

* Change expiration timestamp to not support fractional seconds

* Set retry timeout

* Harmonize expiration timestamp

* Skip import check on credential keys

* Add error check

* Update docs

* Change field description

* Add test case, simplify test

* Add test case, simplify test

* Rename variable

* Generate docs

---------

Co-authored-by: Henrique Santos <henrique.santos@freiheit.com>

stackit/internal/services/objectstorage/credential/datasource.go

@@ -8,7 +8,6 @@ import (
 	"github.com/hashicorp/terraform-plugin-log/tflog"
 	"github.com/stackitcloud/terraform-provider-stackit/stackit/internal/core"
 
-	"github.com/hashicorp/terraform-plugin-framework-timetypes/timetypes"
 	"github.com/hashicorp/terraform-plugin-framework/datasource/schema"
 	"github.com/stackitcloud/stackit-sdk-go/core/config"
 	"github.com/stackitcloud/stackit-sdk-go/services/objectstorage"
@@ -110,8 +109,7 @@ func (r *credentialDataSource) Schema(_ context.Context, _ datasource.SchemaRequ
 				Sensitive: true,
 			},
 			"expiration_timestamp": schema.StringAttribute{
-				CustomType: timetypes.RFC3339Type{},
-				Required:   true,
+				Computed: true,
 			},
 		},
 	}