[Feature] configurable **client** certificate

Hi, I encountered a problem with the client certificate verifying / authenticating.

When enabled client certs (typically the Cloudflare mTLS rules), the client must present its private cert (aka .p12/.pfx file) to the server. However since ChatGPT-Next-Web requests the API base from the backend, it would not carry the valid credentials that should have come from a browser request.

So there should be a configurable option to specify a client certificate so that the mutual verification would work.

I have read the issue list and found that #518 #3034 may be related, but both of them didn't mention if it's possible to deploy a client certificate for the backend.

My current approach is to allow the server IP as request src_ip, but it's quite inconvenient since I had to hard code the IP into the rules. So let me ask for a feature to satisfy this scene.