Idea: Add configure option to require simdjson PECL for faster json decoding

[TysonAndre]

See crazyxman/simdjson_php#86 for an example of how to call it when memcached is configured to use json - this would work when defined(SIMDJSON_PHP_VERSION_ID) && SIMDJSON_PHP_VERSION_ID >= 30000

https://github.com/crazyxman/simdjson_php/pull/86/files#diff-e40ddac25bf760132109ead4b3fd8bbb2ab571a5ed01d7eb841a2c81bf8d28d3

For decoding json, this is faster than php's bison-based parser in various use cases: https://github.com/crazyxman/simdjson_php/tree/master/benchmark#run-benchmark

Is this something you'd be interested in a PR for? (e.g. --enable-memcached-simdjson)