Add absoluteId() to get the absolute ID of an InngestFunction (#804)

## Summary
<!-- Succinctly describe your change, providing context, what you've
changed, and why. -->

Add `InngestFunction#absoluteId()` to fetch the full ID using the
client.

## Checklist
<!-- Tick these items off as you progress. -->
<!-- If an item isn't applicable, ideally please strikeout the item by
wrapping it in "~~"" and suffix it with "N/A My reason for skipping
this." -->
<!-- e.g. "- [ ] ~~Added tests~~ N/A Only touches docs" -->

- [ ] ~Added a [docs PR](https://github.com/inngest/website) that
references this PR~ N/A Used internally
- [ ] ~Added unit/integration tests~ N/A
- [x] Added changesets if applicable

.changeset/wise-rings-marry.md

@@ -0,0 +1,5 @@
+---
+"inngest": minor
+---
+
+Add `InngestFunction#absoluteId()` to get the absolute ID of an `InngestFunction`

packages/inngest/src/components/InngestFunction.ts

@@ -90,6 +90,14 @@ export class InngestFunction<
     return [prefix, this.opts.id].filter(Boolean).join("-");
   }
 
+  /**
+   * The generated or given ID for this function, prefixed with the app ID. This
+   * is used for routing invokes and identifying the function across apps.
+   */
+  protected get absoluteId(): string {
+    return this.id(this.client.id);
+  }
+
   /**
    * The name of this function as it will appear in the Inngest Cloud UI.
    */