feat: Order/disable endpoints with ENDPOINTS env var

[danny-avila]

Summary

In your .env file,

Uncomment the ENDPOINTS variable to determine which endpoints are available.
Listed here are the only accepted values, which are all enabled if an authentication
option is provided, whether user provided or admin provided in this .env file.
Note: the first value is considered the default value.

Also note that clicking on a conversation will make that conversation's endpoint the target endpoint for a new chat, and saves this in local storage.

# ENDPOINTS=openAI,azureOpenAI,bingAI,chatGPTBrowser,google,gptPlugins,anthropic

Also closes #1205

From discussion #1158

Change Type

• New feature (non-breaking change which adds functionality)

Testing

Manual testing

Checklist

• My code adheres to this project's style guidelines
• I have performed a self-review of my own code
• I have commented in any complex areas of my code
• I have made pertinent documentation changes
• My changes do not introduce new warnings
• I have written tests demonstrating that my changes are effective or that my feature works
• Local unit tests pass with my changes
• Any changes dependent on mine have been merged and published in downstream modules.

[zimmra]

@danny-avila this seems to not handle custom endpoints

Observations:

• The custom endpoints are always grouped together in the list
• The custom endpoints adhere to the order they are in within librechat.yaml
• The custom endpoints order within all available endpoints cannot be set, they seemingly completely disregard the ENDPOINTS variable and they 'want' to be at the top of the list. The only thing that overrides this behavior is setting a different natively supported provider in the list first.

You can test this by having a custom provider and typing any random string as your first ENDPOINTS value.
In these examples I have OpenRouter configured as my first entry within librechat.yaml and OpenAI via Copilot as my second entry

• ENDPOINTS=bingAI,google,OpenRouter,OpenAI via Copilot,openAI,gptPlugins
  • This results in the following order: Bing > OpenRouter > OpenAI via Copilot > Google > OpenAI > GPT Plugins
• ENDPOINTS=randomstring,bingAI,google,OpenRouter,OpenAI via Copilot,openAI,gptPlugins
  • This results in the order OpenRouter > OpenAI via Copilot > Bing > Google > OpenAI > GPT Plugins

If I change the order of OpenRouter and OpenAI via Copilot within the librechat.yaml they adhere to that and change places.