[FEAT]: Add a github_copilot_enterprise_seats data source #3730

Describe the need

Enterprise owners and billing managers on GitHub Enterprise Cloud cannot read the Copilot seats billed to their enterprise from Terraform. We want to use that list in Terraform, for example to check seat usage against a budget or to find seats with no recorded Copilot activity.

The REST endpoint List all Copilot seat assignments for an enterprise (GET /enterprises/{enterprise}/copilot/billing/seats, public preview) is already covered by the go-github version the provider uses (v92: ListCopilotEnterpriseSeats and ListCopilotEnterpriseSeatsIter), so no SDK update is needed. #3462 covers organization and team seat assignment, not this enterprise-wide list.

Proposed interface:

data "github_copilot_enterprise_seats" "example" {
  enterprise_slug = "example-co"
}

Computed attributes: total_seats, and seats with assignee_login, assigning_team_slug, plan_type, created_at, last_activity_at, last_activity_editor and pending_cancellation_date.

I can open a PR for this.

GitHub Installation Type

• GitHub.com (Free, Pro, or Team)
• GitHub Enterprise Server (on-premises)
• GitHub Enterprise Cloud with Personal Accounts (github.com)
• GitHub Enterprise Cloud with Managed Users/EMU (github.com)
• GitHub Enterprise Cloud with Data Residency (*.ghe.com)
• I don't know

Relevant log output

N/A

Code of Conduct

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