auth-ext: support OAuth 2.0 providers (#913)

The built-in OAuth 2.0 providers, as well as custom OpenID Connect
providers, are now supported out of the box. Such extension can be turn
on/off manually by e.g.:
* `g.auth.with_google(**ext_settings)`
* `g.auth.with_openid_provider('builtin::oauth_google', **ext_settings)`
(equivalent to the one above)
* `g.auth.without_azure()`
* `g.auth.without_openid_provider('my_custom_oidc_provider')`

or customized with e.g. hooks like:

```python
@g.auth.apple.on_sign_up_complete
async def sign_up_complete(token_data: gel.auth.oauth.TokenData):
    name: str = token_data.get_id_token_claims()["name"]
    ...
```