Document composition function required schemas

[negz]

What's Missing?

Crossplane 2.2 adds support for composition functions to request OpenAPI schemas.

Functions sometimes need OpenAPI schemas for resource kinds - for example to validate resources, generate resources with correct field types, or build schema-aware tooling. Since functions can't assume network access per the function specification, they have no way to fetch schemas directly.

This feature extends the existing required resources pattern to support schemas. Functions return requirements.schemas specifying GVKs they need, and Crossplane populates required_schemas on the next request. If a schema isn't found, Crossplane returns an empty Schema{} to indicate it tried, matching the pattern used for required resources.

See crossplane/crossplane#7022 for details.