Add withProvider() method for runtime provider configuration

[inkarnation]

This PR adds a fluent withProvider() method to agents, allowing on-the-fly configuration of provider credentials (key, URL, driver). This is specifically useful for multi-tenant applications where you need to use a user's own API key or a custom proxy without touching the global app configuration.

Resolves #105.

// 1. Override credentials for an existing config
(new MyAgent)
    ->withProvider('anthropic', key: $tenantKey)
    ->prompt('Analyze this...');

// 2. Custom proxy or alternative URL
(new MyAgent)
    ->withProvider('anthropic', url: 'https://litellm.proxy.local/v1')
    ->prompt('Analyze this...');

// 3. Register a "virtual" provider at runtime
(new MyAgent)
    ->withProvider('tenant-api', driver: 'openai', key: $tenantKey)
    ->prompt('Analyze this...');

Design Decisions

• Only provided arguments override the base config. Calling withProvider('openai', key: '...') will still respect your global openai URL and driver settings.
• An explicit provider passed directly to prompt(provider: '...') will take priority, but it will use that provider's default global config (ignoring the instance-level override).
• Security / Serialization: These overrides are kept in-memory only. They are not serialized. No queue serialization - runtime API keys must never reach Redis/SQS/database. (Exception is created)

[pushpak1300]

fix conflicts ?

[inkarnation]

@pushpak1300 sure, done