Add ExpiresAt field to ProjectSharedWithGroup struct

cindyrequested to merge
gitlab-community/gitlab-org/api/client-go:add-expires-at-to-project-shared-with-group into main

What does this MR do?

Adds the ExpiresAt field to the ProjectSharedWithGroup struct to support reading the expiration date.

Related to: gitlab-org/terraform-provider-gitlab!2855

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