Add support for LLM Gateway w/ Anthropic

[Karrq]

This PR adds support for LLM Gateways as described in anthropic docs https://docs.anthropic.com/en/docs/claude-code/llm-gateway

[Karrq]

For others here, I found that I can have the same functionality without these changes, simply by overriding the provider options in the opencode config:

"provider": {
        "anthropic": {
            "models": {},
            "options": {
                "apiKey": "{env:ANTHROPIC_AUTH_TOKEN}",
                "baseURL": "{env:ANTHROPIC_BASE_URL}/v1"
            }
        }
    }