Add JSON ingestion schema extension modules

[jwils]

Why

Introduce the JSON ingestion extension implementation separately from the branch that makes it the default and removes the old core implementation.

What

• Add schema_definition extension modules under elasticgraph-json_ingestion
• Add the small core hooks/RBS surface those extension modules need
• Keep existing core JSON Schema behavior in place in this layer

Risk Assessment

Medium - this adds new extension code, but it is not yet the default behavior in this PR.

References

• Stacked on Namespace JSON schema helpers under JSON ingestion #1203.
• Local spellcheck, lint, type check, schema artifact check, and dependency diagram verification passed while building this split.