feat: VertexAI keyFile, keyJson, and express/apiKey support

[RomneyDa]

Description

Adds support for additional authentication methods for VertexAI

• Path to keys json file using env.keyFile (YAML only)
• Inline keys json using env.keyJson (YAML only)
• Express mode using apiKey (JSON and YAML)

Adds docs for the express mode case.
TODO add docs for keyFile and keyJson methods

---

Summary by cubic

Added support for VertexAI Express mode, allowing users to authenticate with an API key instead of Google Cloud credentials. Updated documentation with setup instructions and examples.

• New Features
  • Accepts apiKey for Gemini models using Express mode.
  • Ignores region and projectId when apiKey is provided.
  • Added docs with YAML and JSON config examples for Express mode.

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[cubic-dev-ai]

cubic found 1 issue across 2 files. Review it in cubic.dev

_{React with 👍 or 👎 to teach cubic. Tag @cubic-dev-ai to give specific feedback.}

[sestinj]

@RomneyDa I really really really don't like that this is all inlined in the constructor, but gotta get this out. so can refactor later