Instantiate indexing field types through factory

[jwils]

Why

The JSON ingestion extension needs to extend indexing field type instances through the schema definition factory instead of wrapping them separately.

What

• Add factory methods for enum, object, scalar, and union indexing field types.
• Route core to_indexing_field_type call sites through those factory methods.
• Update the factory RBS signatures.

Verification

• bundle exec rspec elasticgraph-schema_definition/spec/unit/elastic_graph/schema_definition/json_schema_spec.rb
• script/type_check
• script/lint
• script/run_gem_specs elasticgraph-schema_definition was not run to completion because the test datastore was not running at http://localhost:9234.

References #1204 (comment).