[Schema] Generate Schema based on official JSON Schema

The official MCP specification has a TypeScript and JSON Schema definition, which was the base for building the Mcp\Schema namespace.

However, we should evaluate if we can generate PHP classes based on the JSON Schema (expected to be easier than Typescript), instead of writing and maintaining that component itself.

There are packages out there for this tasks, and we should check what kind of responsibilities we would need to move away from the Mcp\Schema namespace, if needed.

Please announce yourself before working on this - so we can avoid duplicate effort.

Issue にリンクされている公式の TypeScript および JSON Schema の定義を、既存の Mcp\Schema namespace と比較します。利用可能な PHP JSON Schema 生成パッケージを調査し、どの責務を Mcp\Schema から移行するのかを文書化します。完了とは、特定したパッケージと移行範囲を含む、根拠のある実装計画があることを意味します。