fun deploy 对证书文件检查不严谨

目前的实现方式是判断文件名后缀是否为`.pem`，这样是否不太好？如果要检查证书文件是否正确，应该检查其内容，而不是文件名。

https://github.com/alibaba/funcraft/blob/06373915586924140b24abb7842bb52eae73d07e/src/lib/deploy/deploy-support.js#L253

Start at src/lib/deploy/deploy-support.js around line 253, where deployment currently checks the certificate filename suffix. Trace how the certificate file is read and identify the existing validation entry point; done means validation is based on certificate contents rather than accepting or rejecting files solely by their .pem extension. No test file is mentioned in the issue.

Written by the indexing model from the issue text.