Put common models into a separate package

[emann]

Is your feature request related to a problem? Please describe.
When using multiple generated clients in the same project, things like Client, HTTPValidationError, etc. can be imported from either client package even though they are the same.

Describe the solution you'd like

1. Make a separate package called "openapi-python-client-models" or something similar that includes any models/functionality/etc. that is shared by all generated clients

2. Add an option when generating to omit these common models (by default they should be included) and instead add openapi-python-client-models (or whatever the package is called) as a requirement for the client package

Describe alternatives you've considered
Instead of being configurable, the common stuff could just only live in a separate package that is added as a requirement, but I figure most people likely want everything together in one place.

[dbanty]

I think this will be best covered by part of #171 once broken up. Specifically, supporting generating a single client from multiple documents.

While the clients are all identical today, they won't always be. Some day when we support multiple security mechanisms we'll need generated clients to be dynamic.