feat: add display_name filter to github_external_groups data source

[mattias-fjellstrom]

Summary

Adds an optional display_name argument to the github_external_groups data source. When provided, it filters external groups server-side using the GitHub API's display_name query parameter, reducing the amount of data returned and stored in state.

Closes #3409

Changes

• github/data_source_github_external_groups.go — Added display_name optional string attribute to the schema; passes it to ListExternalGroupsOptions.DisplayName when set; includes display_name in the resource ID for uniqueness
• github/data_source_github_external_groups_test.go — New acceptance tests for both filtered and unfiltered queries (gated by skipUnlessEMUEnterprise)
• examples/data-sources/external_groups/example_2.tf — New example showing filtered usage
• templates/data-sources/external_groups.md.tmpl + docs/data-sources/external_groups.md — Updated documentation with new argument and example

Usage

# Fetch all external groups (existing behavior, unchanged)
data "github_external_groups" "all" {}

# Fetch external groups filtered by display name (new)
data "github_external_groups" "filtered" {
  display_name = "my-group"
}

Notes

• The go-github SDK v85 ListExternalGroupsOptions already supports the DisplayName field — no SDK changes needed
• Fully backwards compatible — display_name is optional and defaults to no filter

[github-actions]

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