Add schema definition extension plumbing

[jwils]

Why

Prepare schema definition for ingestion serializer extensions before moving JSON Schema code into its own gem.

What

• Add schema definition extension_modules plumbing for APIs, factories, and rake tasks
• Apply duplicate extension modules only once
• Replace the rake-task sidecar enforcement option with schema-level schema.enforce_json_schema_version false
• Update project templates, tests, and docs to configure JSON schema enforcement in schema definitions

Risk Assessment

Medium — this touches schema definition setup, but JSON Schema behavior remains in elasticgraph-schema_definition in this PR.

References

• Stacked on Add JSON ingestion gem shell #1199.
• Local spellcheck, lint, type-check, targeted specs, and schema artifact checks passed.

[myronmarston]

For the site examples I think we want this to be false, as we don't want to ever bump the json schema version on them. The generated schema artifacts are git-ignored so it works fine on CI with either false or true, but it may add friction if these examples all inherit the default value of true. For example, if I'm iterating on a site example schema the default of true may force me to manually delete the schema artifacts between runs of be rake site:serve, whereas that was previously not needed.

Can you add schema.enforce_json_schema_version false next to schema.json_schema_version 1 in the site example schemas?

[myronmarston]

This is useful API documentation that previously rendered on our website. Can you copy it over to the new schema.enforce_json_schema_version API?

[myronmarston]

I don't think the changes here are needed--if I revert them, script/type_check still passes.