APEX-54 Stricter type checking for unsafe_transform functions (#222973)

Address https://github.com/elastic/kibana/issues/216061

Adds an indirection layer in the definition of the `transformFn:`, which
forces devs to explicitly define the types of the documents being
transformed.

---------

Co-authored-by: kibanamachine <42973632+kibanamachine@users.noreply.github.com>

examples/eso_model_version_example/server/plugin.ts

@@ -104,9 +104,10 @@ export class EsoModelVersionExample
             changes: [
               {
                 type: 'unsafe_transform',
-                transformFn: (document) => {
-                  return { document };
-                },
+                transformFn: (typeSafeGuard) =>
+                  typeSafeGuard((document) => {
+                    return { document };
+                  }),
               },
             ],
             schemas: {