feat: add GCP Vertex AI platform as provider

[uddhav]

Adds support for Claude, Gemini tool using models from the GCP Vertex AI platform

Models added in this edition

• Anthropic

  1. Claude 3.5 Sonnet v2
  2. Claude 3.5 Sonnet

• Google

  1. Gemini 2.0 Pro Experimental
  2. Gemini 2.0 Flash
  3. Gemini 1.5 Pro

NB

I am aware of the recent contribution on this topic that got reverted from main. Builds failed due to a dependency issue that hasn't been resolved yet. Google's Auth SDK crate also seems to be an in-between state for now. That's why providers/gcpauth.rs exists.

  --- stderr
  thread 'main' panicked at /home/runner/.cargo/registry/src/index.crates.io-6f17d22bba15001f/aws-lc-sys-0.25.1/builder/cc_builder.rs:367:13:
  ### COMPILER BUG DETECTED ###
  Your compiler (cc) is not supported due to a memcmp related bug reported in https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95189. We strongly recommend against using this compiler. 
  EXECUTED: true
  ERROR: 
  OUTPUT: 

Testing

• Apart from tests, I have used it with user and service accounts that have been granted access to Vertex AI platform within their GCP project.
• I could only hand test the GCP metadata approach + the test since I don't have access to a GCP VM or compute provisioned to access Vertex AI for now.
• Tested all above models except Claude 3.5 Sonnet v1 - I don't have access to it but seems to be API compatible with v2.
• Continuing to use it at work while this PR is in progress since it's my only way to access to Claude at work for now.

[uddhav]

Now that Claude 3.7 is out - I can wait for the Anthropic provider to get updated. I don't see that anyone is blocked on missing Vertex AI provider.