feat(preferences): add optional birthDate to PreferenceUpdateItem (#289)

* feat(preferences): add optional birthDate to PreferenceUpdateItem

Add optional birthDate (ISO 8601 date YYYY-MM-DD) to PreferenceUpdateItem
for native birthdate / age-gating flows. Prerequisite for main repo Phase 1.

Made-with: Cursor

* docs(preferences): clarify birthDate formats and enforcement location

Made-with: Cursor

* chore: bump version to 4.154.0

Made-with: Cursor

Author: jmenestr

package.json

@@ -2,7 +2,7 @@
   "author": "Transcend Inc.",
   "name": "@transcend-io/privacy-types",
   "description": "Core enums and types that can be useful when interacting with Transcend's public APIs.",
-  "version": "4.153.0",
+  "version": "4.154.0",
   "homepage": "https://github.com/transcend-io/privacy-types",
   "repository": {
     "type": "git",