feat(datastore): add subscription variables for multi-tenant filtering

[anivar]

This PR adds subscription variables support to DataStore, addressing the long-standing need for server-side subscription filtering in multi-tenant applications. Fixes #9413.

The implementation adds a new subscriptionVariables configuration option that allows passing custom variables to GraphQL subscriptions. This enables filtering at the subscription level rather than receiving all updates and filtering client-side - a critical requirement for multi-tenant SaaS applications where each tenant should only receive their own real-time updates.

The solution integrates cleanly with the existing DataStore sync engine by extending DataStoreConfig with an optional subscriptionVariables field. Variables are processed in SubscriptionProcessor and passed through to buildSubscriptionGraphQLOperation. The implementation supports both static variables (objects) and dynamic variables (functions that receive the operation type).

// Static variables for simple multi-tenant filtering
DataStore.configure({
  subscriptionVariables: {
    Todo: { storeId: 'store-123' }
  }
});

// Dynamic variables based on operation type
DataStore.configure({
  subscriptionVariables: {
    Todo: (operation) => ({
      storeId: 'store-123',
      isNew: operation === 'CREATE'
    })
  }
});

The implementation includes comprehensive production safeguards: deep cloning to prevent mutation side effects on cached values, filtering of reserved GraphQL keywords (filter, owner, limit, etc.), memoization to avoid redundant function calls, and proper error handling for edge cases like circular references and Object.create(null).

This directly addresses the use cases discussed in the issue thread where developers need to filter subscriptions by tenant/store IDs at the server level. Without this feature, multi-tenant apps must receive all updates across all tenants and filter client-side, which is inefficient and potentially insecure.

The changes are fully backward compatible - existing DataStore usage continues to work unchanged. Added 12 comprehensive unit tests covering basic functionality, edge cases, mutation protection, and caching behavior. All existing DataStore tests continue to pass.