API request rate limit

[billygrossman]

First, thanks for making this client library!

Currently, ActiveCampaign sets a rate limit of 5 requests per second. It seems that a 503 response code is returned once this limit is reached. It would be helpful if by default a rate limiter were attached to the Guzzle client instance.

As a suggestion, this is a middleware package that seems promising https://github.com/spatie/guzzle-rate-limiter-middleware.

Thanks!