feat: allow setting base_url for LLM extraction strategy in CLI

[atomlong]

Summary

This PR adds the ability to set the base_url for the LLM extraction strategy in the CLI. This allows users to specify a custom base URL for their LLM provider, which is useful for using self-hosted or alternative LLM endpoints.

List of files changed and why

• crawl4ai/cli.py: Modified to accept the base_url parameter in the extraction_config for the LLM extraction strategy and pass it to the LLMConfig object.

How Has This Been Tested?

I have manually tested this change by:

1. Running the CLI with an extraction_config that includes the base_url parameter.
2. Verifying that the base_url is correctly passed to the LLMConfig object.
3. Confirming that the LLM extraction strategy uses the specified base_url when making requests to the LLM provider.

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• 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
• New and existing unit tests pass locally with my changes