Add JSON ingestion indexing extensions

[jwils]

Why

Keep the JSON ingestion indexing extensions separate from the schema-extension wiring so the indexing behavior is easier to review.

What

• Add JSON ingestion extension modules for indexing field references and field types
• Make core indexing field references and enum/scalar/union field types extendable via the existing MemoizableData helper, matching the object field type
• Keep JSON ingestion behavior in extension modules; no wrapper classes

Risk Assessment

Low to medium - this changes internal indexing value objects to use the existing MemoizableData wrapper so JSON ingestion can attach extension modules, without changing core mapping behavior.

References

• Stacked on Namespace JSON schema helpers under JSON ingestion #1203.
• bundle exec rspec elasticgraph-schema_definition/spec/unit/elastic_graph/schema_definition/json_schema_spec.rb elasticgraph-schema_definition/spec/unit/elastic_graph/schema_definition/json_schema_field_metadata_spec.rb elasticgraph-schema_definition/spec/unit/elastic_graph/schema_definition/indexing/json_schema_with_metadata_spec.rb elasticgraph-schema_definition/spec/unit/elastic_graph/schema_definition/factory_spec.rb passed.
• script/type_check passed.
• script/lint passed.

Stack

Current PR is marked with ->.

• #1199 Add JSON ingestion gem shell
• #1200 Add schema definition extension plumbing
• #1201 Move JSON schema helpers to json ingestion gem
• #1203 Namespace JSON schema helpers under JSON ingestion
• -> #1204 Add JSON ingestion indexing extensions
• #1215 Wire JSON ingestion schema extension modules
• #1205 Use JSON ingestion as schema definition extension
• #1202 Document JSON ingestion schema APIs
• #1224 Migrate JSON ingestion specs into gem
• #1220 Add configurable indexing event decoder