Access to the rate limit

[IonicaBizau]

First, thank you very much for this amazing library! ✨

Would it be possible to expose the rate limit header value in an instance property, so that I know what the current rate limit value is?

Thank you very much!

[jrl84]

@IonicaBizau Added a new class config option called return_as_payload, which will return the callAPI results inside a payload key and also adds the rate limit as a separate key called restore_rate.

This value either comes from the x-amzn-ratelimit-limit header if present or else the default restore_rate. Please note that x-amzn-ratelimit-limit header is used to calculate the restore rate (1 / (res.headers['x-amzn-ratelimit-limit'] * 1)). This way it follows the same logic that is currently implemented in the library with the default restore rates, so in essence the value is the wait time until a new request can be sent.