[FEAT]: Support filtering `github_external_groups` data source using `display_name` argument

### Describe the need

The `github_external_groups` data source currently supports zero arguments.

This is awkward when you have a lot of groups, especially since the result will be persisted to the state file.

The underlying API allows you to filter the results based on `display_name` ([API docs](https://docs.github.com/en/enterprise-cloud@latest/rest/teams/external-groups?apiVersion=2026-03-10#list-external-groups-available-to-an-organization), which would narrow down the results by a lot. The data source should also support this optional argument.

### SDK Version

_No response_

### API Version

_No response_

### Relevant log output

```shell

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### Code of Conduct

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