[GPT-Codemaster] Fixes issue #5: Custom system messages

[github-actions]

This Pull Request was created automatically by GPT-Codemaster
Resolves #5

[dex3r]

/reviewpad run

[reviewpad]

Reviewpad Report

‼️ Errors

• Unconventional commit detected: '[GPT-Codemaster] Progress on issue Custom system messages  #5: Custom system messages' (a09b9a5)
• Unconventional title detected: '[GPT-Codemaster] Fixes issue Custom system messages  #5: Custom system messages ' illegal '[' character in commit message type: col=00

[dex3r]

/reviewpad summarize

[reviewpad]

AI-Generated Pull Request Summary: This pull request includes the changes made to FollowNextStepExecutor.cs to provide support for custom system messages. It adds a _customSystemMessage private variable and a SetCustomSystemMessage() public method that sets the value of this variable. It also modifies the GetSystemMessage() method to return the custom system message if it is set.