Add support for bigints in JSON ( store as string )

[vladpaiu]

Summary
Add support for bigints in JSON

Details
In certain scenarios ( like #3533 ), we are processing JSONs which have bigints in them, so we need to properly parse those and export sub-fields in the opensips scripts.

Solution
Expose bigints in JSON as string to the opensips script.

Compatibility
Backwards compatible

[liviuchircu]

Undefined variable: int64_t int_value;. There is also a double ;; typo in the middle. LGTM otherwise.

[liviuchircu]

Let's merge this, as it fixes a big limitation, at the end of the day. Still, until we have 64bit integer support at opensips.cfg level, script developers will have to deal with the fact that this solution lacks symmetry... i.e. if I re-serialize a freshly parsed $json variable back to a JSON string, the module will not be able to distinguish between a "OpenSIPS-quoted 64-bit integer" or a "string of digits longer than 10".