Improve developer UX for using local evaluation in PHP client

As per the issues #99 & #101, there are a few developer UX issues when using local evaluation in our PHP client which we should try to resolve.

The key issue here is that it is currently required to create a separate cron job to update the environment document cache outside of the integration of the Flagsmith SDK into the customer application. It may not be possible to remove this, but there are improvements that we can make:

1. The documentation is not overly clear - we should provide exact steps on how to achieve a working set up of local evaluation mode in PHP
2. The functionality to enable local evaluation mode relies on the `environmentTtl` property which, when set in the client itself and not in the cron job, seemingly has no effect other than to enable local evaluation mode. We should look to bring this functionality more in-line with the rest of the clients, and add an `enableLocalEvaluation` argument.