Update extension requirements

[nCrazed]

As far as I can tell ever since #31 ext-curl is not used in the default configuration. As such it should be marked optional.

[nCrazed]

Similarly ext-sockets should be added (although it is currently unclear to me if it's required for API call use cases.

While PHP usually compiled with sockets by default, it is not the case in docker environments where using this package without explicitly enabling the extension will crash the application at run-time.

[SammyK]

Thanks for the contribution @nCrazed! Sorry for the delay on responding to this.

I'm all for moving curl to a suggestion. Before we do, we'll need to handle the case where curl is not installed and the user has not configured UDP. Perhaps we check for extension_loaded('curl') here and possibly fall back to UDP if curl is not installed? There might be better options so if you have a better idea, feel free to chime in. :)