This PR kicks off implementation of v2 client by establishing a new, long-lived branch and reworking the way API requests are made. Note that the overall design decision is to make the client thinner.

Below is the list of the most important changes:

• axios is replaced by cross-fetch wrapped with strongly-typed API client stub generated from the OpenAPI schema
• Caching is done internally by the fetch API, driven by HTTP headers
• Retry functionality is now provided by fetch-retry, promise-retry wasn't really pluggable into the stub system.
• Errors have been reworked. Namely RequestError has been made abstract. New, more specific, NetworkErrorhas been added and RequestCanceledError has been removed entirely as the user supplying the AbortSignal should be able to handle the abort error.
• API types are no longer exported, OpenAPI schema is a superior replacement.
• Client types are no longer exported, users are advised to use typescript type utilities to extract them (e.g. Parameters<typeof client.models>, Awaited<ReturnType<typeof client.models>>). We are open to reintroducing them if this proves to be insufficient.
• Support for Node v16 has been dropped, we're now targeting v18.12.0 and higher. This is to properly support Lanchain and Fetch utilities in the generated code. Note that we could still make it work with Node v16 if there will be complaints from users.
• zod schemas have been dropped as it's maintenance would require huge effort and it would duplicate type generation.