[Feature] Multiple base URLs for different models

**Is your feature request related to a problem? Please describe.**
It would be useful to be able to use one frontend for multiple backends. For instance, Being able to use OpenAI models as well as local models could prevent one from using multiple instances of the same website.

**Describe the solution you'd like**
A possible solution would be to be able to provide a base URL for each model in the CUSTOM_MODELS variable used when launch a nextchat docker container. For instance: `CUSTOM_MODELS='+all,+local-model-1[https://local-api-1.website],+local-model-2[https://local-api-2.website]'`. For each model that doesn't have a specified URL, the BASE_URL is used instead.

**Describe alternatives you've considered**
Running multiples instances of the website under different subdomains and multiple nginx server configurations.