add changeset for shortId optimization change (#21944)

## Description

This PR adds a changeset for the change that optimizes `Tree.shortId`
api from [this
PR](https://github.com/microsoft/FluidFramework/pull/21822)

.changeset/many-steaks-mix.md

@@ -0,0 +1,8 @@
+---
+"fluid-framework": minor
+"@fluidframework/tree": minor
+---
+
+tree: Improved performance for accessing identifiers in shortId API
+
+Users should see improved performance when calling the `Tree.shortId` API. Identifier field keys are now cached in the schema for faster access.