[TJKoury]

JSON Schema import/export

This PR makes JSON Schema a first-class schema format for flatc, while still flowing through the existing FlatBuffers schema IR (reflection/reflection.fbs).

What’s included

JSON Schema export: flatc --jsonschema generates *.schema.json from .fbs
JSON Schema import: treat *.schema.json as a schema input (IDL-like), map into the FlatBuffers schema IR, and run normal generators
Optional lossless round-tripping: --jsonschema-xflatbuffers emits x-flatbuffers vendor-extension metadata + meta-schema docs

Docs + tests:

Generation + golden + round-trip stability: tests/JsonSchemaTest.sh
Import “wild” JSON Schema/OpenAPI fixtures + stable regeneration: tests/JsonSchemaImportTest.sh (fixtures under tests/jsonschema_import/)
Usage examples
Export FlatBuffers → JSON Schema: flatc --jsonschema -o out schema.fbs
Export (lossless): flatc --jsonschema --jsonschema-xflatbuffers -o out schema.fbs
Import JSON Schema → generators: flatc --cpp -o out schema.schema.json (override root via --root-type when needed)

Implementation notes / review tips

CLI wiring + .schema.json input detection: src/flatc.cpp
JSON Schema importer: Parser::DoParseJsonSchema in src/idl_parser.cpp
JSON Schema generator: src/idl_gen_json_schema.cpp
x-flatbuffers meta-schema: docs/source/schemas/x-flatbuffers.schema.json
Testing

Passed testing locally:

ctest (runs flattests)
tests/CppTest.sh
tests/JsonSchemaTest.sh
tests/JsonSchemaImportTest.sh
tests/PythonTest.sh
Go tests (default + --preserve-case)
tests/PHPTest.sh (default + --preserve-case)

The importer is intentionally permissive (unknown JSON Schema/OpenAPI keywords are ignored).
Full fidelity requires x-flatbuffers metadata; without it, some FlatBuffers semantics can’t be expressed in plain JSON Schema.