Return "public_email" for gitlab_user data source

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The value is supported by the API if you're a reuglar user: https://docs.gitlab.com/api/users/#retrieve-a-single-user-as-a-regular-user

But the respective data source does not return/expose this value: https://gitlab.com/gitlab-org/terraform-provider-gitlab/-/blob/main/internal/provider/datasource_gitlab_user.go?ref_type=heads#L38

Provider Version is 18.8.2

Implementation Guide

• Follow the CONTRIBUTING.md guide to set up your local development environment.
• Clone the community fork of this project.
• In internal/provider/datasource_gitlab_user.go, add a new Computed attribute called public_email. Set the value in the data struct from the API response.
• Update the description of the existing email attribute to make it clear that this is for searching and does not necessarily represent the public email.