Add Symfony Deprecation Contracts package

- Created CHANGELOG.md to maintain version history.
- Added README.md with usage instructions for the trigger_deprecation() function.
- Initialized composer.json for the Symfony Deprecation Contracts library, specifying dependencies and autoloading.

modules/pshowsso/deps/symfony/deprecation-contracts/CHANGELOG.md

@@ -0,0 +1,5 @@
+CHANGELOG
+=========
+
+The changelog is maintained for all Symfony contracts at the following URL:
+https://github.com/symfony/contracts/blob/main/CHANGELOG.md