Remove all function_exists for extension checking on Client class

[peter279k]

As title, it seems that Client class do many function_exists functions to check whether required PHP extensions are loaded.

Such as curl, json and so on.

I think it's time to be removed and it can also move them to require block on composer.json.

And it can check extensions are loaded correctly during composer install command running.

Once this issue is accepted by @xperseguers, I would be happy to work on this :).

[xperseguers]

Feel free to create a PR.