Add support of external references in JSON schema

:Release Notes:
Add support of external references in JSON schema.
Add C API jvalue_tostring_schemainfo with support of external resolver.

:Detailed Notes:
The schema parser did not resolve external references, as a result
schema validation failed to validate fields from externals.

:Testing Performed:
Checked using internal unit tests
Starfih-image was built and checked on virtualbox

:QA Notes:

:Issues Addressed:
[GF-4639] pbnjson "$ref" and "extends" keywords not working

Open-webOS-DCO-1.0-Signed-off-by: Alekseyev Oleksandr <alekseyev.oleksandr@lge.com>

Change-Id: I6a6882efe42d2c3abaf576880e3a15c6762dcbfd
Reviewed-on: https://g2g.palm.com/2840
Reviewed-by: Build Verification
Reviewed-by: Alekseyev Oleksandr <alekseyev.oleksandr@lge.com>
Tested-by: Alekseyev Oleksandr <alekseyev.oleksandr@lge.com>
Reviewed-by: Anatolii Sakhnik <anatolii.sakhnik@lge.com>
Tested-by: Anatolii Sakhnik <anatolii.sakhnik@lge.com>