refactor: Consolidate public interface for v3 (#794)

### Description

Consolidates the public interface for the v3 release:
- Root `apify_client` namespace now only exposes `ApifyClient`,
`ApifyClientAsync`, and `__version__`. This is a go-to for everyone.
- HTTP client abstractions moved to `http_clients` submodule
(`HttpClient`, `HttpClientAsync`, `HttpResponse`, `ImpitHttpClient`,
`ImpitHttpClientAsync`). Only for people who want to replace the default
HTTP client.
- New public `types.py` module with `Timeout`, `WebhooksList`, and
`JsonSerializable` type aliases (can be used, e.g., for a custom HTTP
client).
- Add missing `__all__` to `errors.py`.
- Update v3 upgrade guide and doc snippets to reflect new import paths.
- Not breaking, because HTTP clients were not part of v2.5.

I would prefer to keep `_models` and `_typeddicts` private. They should
not be considered safe to import, since exposing them publicly would
potentially restrict future API changes. For typed dicts, users do not
need to import them to pass inputs to resource client methods. Resource
clients, which are also private and users should be able to get to them
only via `ApifyClient` (and its async version), may return Pydantic
models, but those models should not be constructed directly in user
code. There may be niche cases where users want to import models or
typed dicts for explicit typing or casts, but I would rather keep those
available only on the user's own responsibility - we cannot guarantee
that the types will not change. This is also a status quo in the current
v2.5.

This is my stand, let me know yours @janbuchar.

### Issue

Closes: #744