Extract JSON ingestion into a dedicated gem

[jwils]

Why

PR #1079 mixed extraction, new config surface area, and test movement into one large change. This first step isolates the JSON-ingestion extraction while preserving the existing schema_definition behavior and test surface as much as possible.

What

• Add the new elasticgraph-json_ingestion gem with the extracted JSON schema ingestion implementation and tests
• Keep elasticgraph-schema_definition auto-wired for JSON ingestion and preserve old require paths via compatibility wrappers
• Drop the new serializer-extension config surface from this first PR and keep the old API shape

Risk Assessment

Medium. This moves schema-definition internals and artifact generation code, but preserves the existing public behavior and is covered by the extracted gem specs plus the preserved schema_definition unit and integration suites.

References

• Prior feedback split target: Pluggable ingestion serializers with json_ingestion #1079
• Local verification: extracted gem specs, preserved schema_definition JSON schema specs, and schema_definition rake_tasks integration spec passed

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