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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El issue no menciona archivos, pruebas ni puntos de entrada. Empieza por aclarar cómo debería publicarse y referenciarse desde la configuración de una función un recurso de runtime independiente administrado por el cliente y, después, define los flujos del publisher y del consumer del runtime. Se considera terminado cuando los runtimes administrados por el cliente puedan crearse, identificarse mediante su ARN, ser consumidos por funciones y actualizarse para usuarios a escala.

Escrito por el modelo de indexación a partir del texto del issue.