Create your own managed runtime (with custom dependencies, libraries, tools)

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**Description**
Add support for customer-managed runtimes as a standalone resource type. This introduces a new persona, the 'runtime publisher'. For example, an enterprise platform team could publish their own runtimes for use in their organization, packaging shared components such as security and monitoring tools in the runtime. Application developers can consume the runtime in their functions in the same way as a Lambda-managed runtime, simply by specifying the runtime identifier (in this case, an ARN) in their function configuration. The runtime publisher manages updates to the runtime, enabling changes to shared components to be pushed out at scale without requiring per-function updates.

**Additional context**
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Issue にはファイル、テスト、エントリーポイントが記載されていません。まず、スタンドアロンの顧客管理ランタイムリソースをどのように公開し、Function の設定から参照するべきかを明確にし、次にランタイムの publisher と consumer のフローを定義してください。顧客管理ランタイムを作成し、ARN で識別し、Function で使用し、大規模なユーザー向けに更新できれば完了です。

索引モデルが issue の本文から書いたものです。