feat: Add system promt support for beta

[davidgustys]

This is almost 1:1 copy of @gregpriday PR:https://github.com/google-gemini-php/client/pull/42, but added on beta branch instead of main.

This PR introduces system instruction support and adds the ability to easily configure the client to use the beta API endpoint (required for system instructions).

Changes

1. System Instruction Support:

   • Added withSystemInstruction method to the GenerativeModel class
   • Updated GenerateContentRequest and related components to include system instructions in API requests
   • Modified Client and GenerativeModelContract to support system instructions
   • Added tests to verify system instruction functionality

2. Added tests to cover the new beta API configuration

Additional Notes:

• The code follows the project's coding style (verified using composer lint)
• Tests have been added for the new functionality
• The commit history is coherent and meaningful
• No breaking changes have been introduced, maintaining SemVer compatibility
• Tested in my own project, and everything is working properly
• Please review and let me know if any further changes are needed.

[Plytas]

Hey @aydinfatih, I have adjusted the PR according to your comments. Also ran composer test and it's all green now.