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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Nell’issue non sono indicati file, test o punti di ingresso. Inizia chiarendo come una risorsa runtime autonoma gestita dal cliente debba essere pubblicata e referenziata dalla configurazione della funzione, quindi definisci i flussi del publisher e del consumer del runtime. Il lavoro è completato quando i runtime gestiti dal cliente possono essere creati, identificati tramite ARN, utilizzati dalle funzioni e aggiornati per gli utenti su larga scala.

Scritto dal modello di indicizzazione a partire dal testo della issue.