Requiring ext-json

[mihai-stancu]

Hello,

For Composer based usage specifying ext-json in composer.json require field would be better than the runtime validation used in AbstractChargeBee, allowing composer to check for the presence of the extensions at install time rather than seeing the error at runtime.

https://github.com/chargebee/chargebee-php/blob/6d9456181532543e9fc0e77ca0ae347b836da433/lib/AbstractChargeBee.php#L7 https://github.com/chargebee/chargebee-php/blob/6d9456181532543e9fc0e77ca0ae347b836da433/composer.json#L8

I can submit this as a PR if relevant.