Feature/provider base url

[Sacristaan]

Summary

Please include a summary of the change and/or which issues are fixed.

Fixes #1082 https://github.com/unclecode/crawl4ai/discussions/1082

List of files changed and why

All files related with api_key_env, adding base_url_env

How Has This Been Tested?

Deploying the docker image and pointing the base_url to an Ollama instance

Checklist:

• [x] My code follows the style guidelines of this project
• [x] I have performed a self-review of my own code
• [x] I have commented my code, particularly in hard-to-understand areas
• [x] I have made corresponding changes to the documentation
• [ ] I have added/updated unit tests that prove my fix is effective or that my feature works
• [x] New and existing unit tests pass locally with my changes