fix: improve v2 callable typing

[IzaakGough]

Fixes #257

[gemini-code-assist]

Code Review

This pull request refactors the TypeScript types for wrapping v2 callable Cloud Functions to support generic input and return types (T and Return), replacing the previous usage of any. It introduces a new WrappedV2CallableRequest<T> type to represent the wrapped request object. The reviewer feedback suggests several improvements to enhance usability and safety: making the data property optional when the input type T can be undefined, renaming the parameter of WrappedV2CallableFunction from data to request (and making it optional), and safely handling potential undefined request arguments at runtime within wrapV2 to prevent crashes.