[OAS] Allow custom sorting/post-processing of OAS properties generated from router

The OAS converter currently emits properties in the order returned by joi-to-json, with no way to apply custom sorting for user-facing OAS docs.

Current behavior:

• Properties appear in the order provided by joi-to-json.
• OpenAPI/OAS has no built-in support for custom field ordering.
• Modifying the generated YAML is possible but will be overwritten whenever the generator runs.

Possible solutions

• Add a post-processing hook (similar to postProcessMutations in parse.ts) or an extension point (e.g., ObjectTypeOptionsMeta) so we can reorder properties in the final OAS output (not limited to alphabetical sorting).
• Alternatively explore safe ways to modify final YAML output without being clobbered by regeneration.
• Something completely different

References:

• Example hook: parse.ts postProcessMutations — https://github.com/elastic/kibana/blob/6f6a42d492aea851c3aaf68830e667491a4660fe/src/platform/packages/shared/kbn-router-to-openapispec/src/oas_converter/kbn_config_schema/parse.ts#L38-L52

Expected outcome:

• A pluggable mechanism to apply custom sorting or other transformations to generated OAS properties before writing the final spec.