POC: client packages

[mrlubos]

Let's start with blank packages configured similarly to openapi-ts. Try to move as much from core folder as possible. See where it starts breaking and let's discuss. Versioning should follow openapi-ts so when people install them both, it's easy to see in their dependencies whether they're out of sync. I see other libraries do releases like this and for the untouched package changelog generally says "bump only for "

We could start with client-fetch, client-axios, and client-next