Determine a way for other modules to extend the Experience builder API spec.

Just follow the example set in 📌 Personalization Segment internal HTTP API integration Postponed last week 😊🙏

Let's fix this in 🐛 External AI Chatbot Functionality Active itself.

Actually, repurposing this to solve the general problem, since that's not specific to the (upcoming) AI submodule.

See https://techdocs.studio/blog/how-to-split-open-api-spec-into-multiple-files for an example.

I expect:

1. /openapi.yml to continue to exist, but for it to use $ref to include other files
2. /modules/xb_personalization/openapi.yml to be added
3. /modules/xb_ai/openapi.yml to be added (original scope of this issue)

That would solve the "submodules add API routes" problem.

Bonus points: splitting /openapi.yml into:

1. /openapi.config.yml (for experience_builder.api.config.* routes)
2. /openapi.content.yml (for experience_builder.api.content.* routes)
3. /openapi.app.yml (for all other experience_builder.api.* routes)
4. /openapi.drupal.yml (for all other experience_builder.* routes)