[Feature] Support self-hosted ShareGPT instance

**Is your feature request related to a problem? Please describe.**

Currently, this app will directly use the official sharegpt instance, and the URL `sharegpt.com` and `shareg.pt` is hardcoded.

Is it possible to add an environment variable to allow users to use custom sharegpt instance? The official instance is a good default option but some users may not prefer to store their data on their instance.

**Describe the solution you'd like**

Add an environment variable like `SHAREGPT_URL` that indicates the instance to use, default to `sharegpt.com`.

**Describe alternatives you've considered**

**Additional context**