Add HTTP client configuration support for Spring AI models

[Fottas]

DeepSeek doesn't support configuring HTTP client settings like connect/read timeout, SSL bundle, redirects. All settings are hardcoded or use JDK defaults.
Proposal: extend DeepSeekConnectionProperties with timeout/SSL config, add RestClientCustomizer to apply settings.

[Fottas]

Update: Extended to All Providers
Initially, I discovered this timeout issue while using DeepSeek and found it was a framework-wide problem affecting multiple model providers.
The PR #4970 extends HTTP client configuration support (timeouts, SSL, proxies, connection pools, etc.) to all applicable model providers, not just DeepSeek.
Implementation Details
Configuration support varies by provider based on their HTTP client implementation:

• RestClient-based providers: Use unified configuration approach
• Custom SDK providers: Configuration depends on their specific SDK capabilities

Result

• Multiple providers now support configurable HTTP client settings
• Backward compatible - all properties are optional