Include ServerCapabilities in server.json

Feels like a nice-to-have, but it would make sense to allow for [ServerCapabilities](https://github.com/modelcontextprotocol/modelcontextprotocol/blob/623e37a3542ceaae0defe1a8450598b90ebbfbf2/schema/2025-03-26/schema.ts#L228) to be included in `server.json` data that can get surfaced back to API consumers.