feat: Refactor external groups data source

[stevehipwell]

Related to #3537

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Before the change?

• github_external_group data source didn't use max per page resulting in a slow query time
• github_external_group didn't support filtering results
• github_external_group had no tests

After the change?

• github_external_group has been refactored to the latest patterns
  • Use max per page
  • Add display_name_filter to filter returned groups
  • Updated documentation
  • Acceptance tests

Pull request checklist

• Schema migrations have been created if needed (example)
• Tests for the changes have been added (for bug fixes / features)
• Docs have been reviewed and added / updated if needed (for bug fixes / features)

Does this introduce a breaking change?

Please see our docs on breaking changes to help!

• Yes
• No

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[github-actions]

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