Remove private json package in favor of encoding/json

[donatwork]

PR Submission checklist

Remove the older internal version of the json package and instead use the now mature and maintained encoding/json package. This was a drop in replacement as the internal code was from an earlier version of encoding/json. There was one customization of importance which relied on a custom unmarshall directive to prevent marshalling the ID field but still allow unmarshalling of the field. I did not see a modern way of doing that, other than a custom unmarshaller. Given that it is standard practice to separate the request and response structures I opted to use a separate structure, ExportReq for (PUT, POST) requests and the normal Export for responses. Unit tests pass and I ran cert-csi provisioning and volume IO tests.

GitHub Issues

GitHub Issue #
dell/csm#1559

Common PR Checklist:

• Have you made sure that the code compiles?
• Have you commented your code, particularly in hard-to-understand areas
• Did you run tests in a real Kubernetes cluster?
• Have you maintained backward compatibility

Description of your changes:

• Removal of api/json
• Separate request and response structures
• Add conversion function to limit northbound changes
• Update of unit tests, remove obsolete tests.