Add support for external schema references for json

[shiveshr]

Problem description
For complex schemas, users may have broken the schema into multiple smaller schemas that are referenced in the complex object. And they may want to break it into multiple smaller files. Json schema description inherently supports this by way of "$ref" which can refer to another schema in the same definitions or in a different location identified by a uri. A uri could be a file path in same directory or another location.

Currently schema registry allows for schemas to be sent as a singular schema file. This works when all the references are defined in the same file. But does not support multiple files.

Problem location
JsonSchema

Suggestions for an improvement
Explore possibility of adding references to json by referencing other schemas registered in the group