[FEAT]: Support team/user slugs in github_repository_environment reviewers

### Describe the need

Right now, the `github_repository_environment` resource only accepts **numeric IDs** for reviewers:

```hcl
reviewers {
teams = [data.github_user.devops.id]
}
```

It would be much easier if it supported **team slugs** and **user logins** instead:

```hcl
reviewers {
teams = ["devops", "security"]
users = ["jdoe", "asmith"]
}
```

This would make configurations more readable and portable, without needing extra data sources like:

```hcl
data "github_team" "devops" {
slug = "devops"
}
```

### SDK Version

_No response_

### API Version

_No response_

### Relevant log output

```shell

```

### Code of Conduct

- [x] I agree to follow this project's Code of Conduct