feat: add support for Poe and Longcat API endpoints

[elifarley]

This pull request adds support for two new services (Poe and Longcat), in addition to the existing OpenRouter integration. Users can now select the desired backend using the new --poe and --longcat options. The script adapts its environment variable checks, API endpoints, and metadata output based on the chosen service.

New Service Flags:

--poe: Use Poe API endpoint (https://api.poe.com/v1/chat/completions)
--longcat: Use Longcat API endpoint (https://api.longcat.chat/openai/v1/chat/completions)

Note: The --provider flag is now restricted to OpenRouter (shows a warning for other services).

Environment Variable Checks:

Requires POE_API_KEY for Poe and LONGCAT_API_KEY for Longcat.

Example for LongCat

export LONGCAT_API_KEY="your-api-key-here"

ask --longcat "Write a Python function to calculate Fibonacci numbers"

# Pipe content to analyze with Longcat
cat mycode.py | ask --longcat "Review this code for bugs"