feat(ollama): allow setting base_url and api_key programmatically

[majiayu000]

Summary

Allow users to configure base_url and api_key for the Ollama provider programmatically, enabling support for multiple Ollama instances and proxied/authenticated deployments.

Fixes #1070

Changes

• Added OllamaApiKey type with optional Bearer token auth (no auth by default)
• Changed OllamaBuilder::ApiKey from Nothing to OllamaApiKey
• from_env() now reads optional OLLAMA_API_KEY env var and defaults OLLAMA_API_BASE_URL to http://localhost:11434 (previously panicked when unset)
• Implemented From<Nothing>, From<String>, From<&str> for full backward compatibility

Usage:

// No auth (unchanged)
let client = ollama::Client::new(Nothing).unwrap();

// With auth + custom URL
let client = ollama::Client::builder()
    .api_key("my-secret-key")
    .base_url("http://remote-ollama:11434")
    .build()
    .unwrap();

Test plan

• All 14 existing Ollama tests pass (including test_client_initialization with Nothing)
• Examples using .api_key(Nothing) compile without changes
• cargo fmt clean
• cargo clippy clean