feat: Added support for Vertex AI for LLM Extraction Strategy

[KennyStryker]

Summary

Fixes #112 - Added support for Vertex AI for LLM Extraction Strategy

List of files changed and why

• crawl4ai/async_configs.py - Added configuration to load creds for Vertex AI.
• crawl4ai/config.py - Added map for vextex ai and its creds.
• crawl4ai/extraction_strategy.py - Merge llm extra kwargs with main extra kwargs.
• deploy/docker/c4ai-code-context.md - Documentation update

How Has This Been Tested?

Manually verfied the results.

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