Extend DiscoveryBase with api_factory setting

[tony-romanovych]

Currently, there's no simple way to adjust internal API client behavior while using discovery at the same time. To do that, one has to somehow override DiscoveryBase._configure_api_client, that is creating new API instances (e.g. BasicApi, BatchApi).

This PR adds optional config setting - api_factory. If present, it's expected to be a callable constructing new API instances. It will be used instead of default instantiation logic. These changes are not breaking the public interface of Discovery classes or Client.

Why:

My primary goal was to make API exception handling more clean, because:
a) There's a bunch of generated ApiException classes: if I work with multiple crm objects, I have to import all those exception classes, give them aliases, and I cannot catch them with single except ApiException
b) There's no way currently to specifically handle only 404 with a single row, e.g. except NotFoundException.
This syntax can be achieved though by extending ApiClient.

from hubspot.crm.deals.exceptions import ApiException as DealsApiException
from hubspot.crm.contacts.exceptions import ApiException as ContactsApiException

try:
    call_hubspot_api()
except DealsApiException as e:
    if e.status == 404:
        ...
    else:
        raise