feat(billing): add data sources for Advanced Security active committers (enterprise + org)

[vmvarela]

Description

Add two data sources to expose GitHub Advanced Security (GHAS) active committer counts, useful for license management and compliance reporting:

• github_enterprise_billing_advanced_security — GHAS active committers for an enterprise
• github_organization_billing_advanced_security — GHAS active committers for an organization

go-github v84 API

func (s *BillingService) GetOrganizationAdvancedSecurityActiveCommitters(ctx context.Context, org string, opts *ActiveCommittersListOptions) (*ActiveCommitters, *Response, error)
// GET /orgs/{org}/settings/billing/advanced-security

Note: Enterprise variant uses the same ActiveCommitters struct — check if go-github v84 exposes an enterprise endpoint or if it must be called via custom request.

Structs

type ActiveCommitters struct {
    TotalAdvancedSecurityCommitters *int
    TotalCount                      *int
    Repositories                    []*AdvancedSecurityRepository
}
type AdvancedSecurityRepository struct {
    Name            *string
    AdvancedSecurityCommitters *int
    AdvancedSecurityCommittersBreakdown []*AdvancedSecurityCommittersBreakdown
}

Acceptance Criteria

• github_organization_billing_advanced_security data source implemented
• github_enterprise_billing_advanced_security data source implemented (verify endpoint availability)
• Schema: owner/enterprise_slug, pagination options
• Computed: total_advanced_security_committers, total_count, repositories list
• Both registered in provider.go
• Docs for each