feat: add first-class support for additional LLM providers

[royalkuriyakosem]

Area

SDK

Problem or motivation

While Opfor supports several popular providers, many organizations deploy
models through platforms such as AWS Bedrock and Google Vertex AI. These
platforms have their own authentication mechanisms and request formats, making
them difficult to use through the current provider options.

First-class support would simplify evaluating agents deployed in enterprise
environments.

Proposed solution

Add provider implementations for:

• AWS Bedrock
• Google Vertex AI
• IBM watsonx.ai
• Cloudflare Workers AI

Each provider should integrate with the existing provider abstraction so users
can select it from the CLI or configuration file without modifying evaluators.

[jithin23-kv]

Thanks for raising this. To be clear, this isn't a support gap — the provider system is already designed to be extensible for exactly this, and it's documented under "Adding an LLM provider" in CONTRIBUTING.md (providerRegistryData in core/src/providers/factory.ts). It's just that Bedrock/Vertex/watsonx/Workers AI haven't been added yet. Keeping this open as help wanted — happy to review a PR for one provider at a time.