fix: add composer configuration and delete DifyClient->file_client

[jayeeliu]

Summary

Optimize sdks/php-client.

1. Add composer configuration to try it quickly
2. Fix syntax error and variable does not exist error in dify-client.php
3. Delete DifyClient::file_client, all requests use DifyClient::client
4. Add composer in README.md

Checklist

Important

Please review the checklist below before submitting your pull request.

• This change requires a documentation update, included: Dify Document
• I understand that this PR may be closed in case there was no previous discussion or issues. (This doesn't apply to typos!)
• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• I've updated the documentation accordingly.
• I ran dev/reformat(backend) and cd web && npx lint-staged(frontend) to appease the lint gods