Add appDowngradeAccessScopes to downgrade write scopes

Shopify now exposes the appDowngradeAccessScopes mutation, which allows an app to replace granted optional write access scopes with their corresponding read scopes. The mutation accepts a non‑null list of write scope handles (e.g., "write_products") and runs on the calling app’s own installation. Only optional write scopes can be downgraded; required scopes are ignored and any write scope not currently granted is skipped. Existing read permissions for those scopes are retained, unlike appRevokeAccessScopes, which would remove matching reads if the write scope was the sole grant. The response payload lists the downgraded AccessScope objects and any user errors. Errors surface via the AppDowngradeAccessScopesUserError type, with codes such as APPLICATION_CANNOT_BE_FOUND, APP_NOT_INSTALLED, UNKNOWN_SCOPES, CANNOT_DOWNGRADE_NON_WRITE_SCOPES, CANNOT_DOWNGRADE_REQUIRED_SCOPES, and CANNOT_DOWNGRADE_UNDECLARED_SCOPES. This change gives developers a safer way to reduce app permissions while preserving read access.