Add JSON ingestion schema support

[jwils]

Why

JSON ingestion needs to live outside the core schema definition gem so applications can opt into JSON Schema artifact generation through the same extension-module mechanism used elsewhere in ElasticGraph.

Stacked on #1220.

What

• Add the elasticgraph-json_ingestion gem shell.
• Add schema-definition extension-module plumbing.
• Move JSON Schema helper types into the JSON ingestion gem namespace.
• Keep JSON schema version enforcement configurable from schema definitions and site examples.

Verification

• script/type_check
• script/run_gem_specs elasticgraph-json_ingestion