feat(billing): add data sources github_organization_billing_packages and github_organization_billing_storage

[vmvarela]

Description

Add two read-only data sources for classic billing at the organization level:

• github_organization_billing_packages — free/paid storage for GitHub Packages
• github_organization_billing_storage — estimated storage used by Actions artifacts and Packages

These are part of the classic billing platform (pre-enhanced billing) and useful for orgs not yet on the enhanced platform.

go-github v84 API

func (s *BillingService) GetOrganizationPackagesBilling(ctx context.Context, org string) (*PackagesBilling, *Response, error)
// GET /orgs/{org}/settings/billing/packages

func (s *BillingService) GetOrganizationStorageBilling(ctx context.Context, org string) (*StorageBilling, *Response, error)
// GET /orgs/{org}/settings/billing/shared-storage

Structs

• PackagesBilling: TotalGigabytesBandwidthUsed, TotalPaidGigabytesBandwidthUsed, IncludedGigabytesBandwidth
• StorageBilling: DaysLeftInBillingCycle, EstimatedPaidStorageForMonth, EstimatedStorageForMonth

Acceptance Criteria

• Two data sources implemented with owner (required) and computed output fields
• Both registered in provider.go
• Docs for each under website/docs/d/
• Acceptance tests