Adding support for `x-include-refresh-token-hard-expires-in`

Hello!

Regarding: https://medium.com/intuitdev/track-your-refresh-tokens-hard-expiry-with-a-new-response-field-7a95ed3ab67f

I have built a wrapper around `intuitlib.client.AuthClient` that subclasses both `refresh` and `get_bearer_token` with adding a new param `include_refresh_token_hard_expires_in=False`. When set to `True` is adds the necessary header field. It also adds `self.x_refresh_token_hard_expires_in = None` to the init.

Would you be open to a pull request to get this into the official version? I see in the blog it's *coming soon*. Not sure if that means it's already in the works or not.

Thanks.

Start with intuitlib.client.AuthClient and inspect the refresh and get_bearer_token methods described in the issue. Compare their existing parameters and response handling with the requested include_refresh_token_hard_expires_in option; done means the option adds the required header and exposes the returned value through x_refresh_token_hard_expires_in.

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