Add -output-json-schema flag for displaying full json schema #2432

Please describe your feature request:

When using -json flag, until the tool is run, output json structure is unknown. Further, many fields in output are optional, and cannot be known beforehand. These fields only show up with certain hosts/input, which makes it difficult to parse result.

Describe the use case of this feature:

When someone wants to parse the json, all the fields and their types are unknown. Using -output-json-schema allows user to view full json schema. This json schema is standard, and can be used by devs to write the types in their own languages/runtimes.