[s3] Add client_config configuration setting

[jschneier]

Closes #1337

[mgzwarrior]

I'm thinking that there is an unintended logical change here that makes self.client_config a required field, defaulted to None. See here for more - #1410

[jschneier]

See the logic here.

[mgzwarrior]

I don't think I am following. The get_default_settings() method here seems to return {} by default.

In our case, we have overridden this method as such:

    def get_default_settings(self):
        return {
            "access_key": setting(f'{self.setting_prefix}_ACCESS_KEY_ID'),
            "secret_key": setting(f'{self.setting_prefix}_SECRET_ACCESS_KEY'),
            ...,
        }

However, we don't have a setting for client_config because this is new. This seems to be the root cause of the error: object has no attribute 'client_config'. One solution I could try is to add a dummy attribute to the above method and default to None, but I shouldn't have to if the setting is not required, right?

[jschneier]

I don't provide backwards compat guarantees on get_default_settings as that is not a public API. If you want this to reliably work I'd recommend something like

settings = super().get_default_settings()
...your changes here