Fixed GH-3526, Fixed the error in using the Builder in SystemPromptTemplate.

[sunyuhan1998]

As mentioned in the issue, currently when using the Builder in SystemPromptTemplate, it returns a Builder of PromptTemplate. This causes subsequent operations like createMessage based on the Builder class to actually return a UserMessage instead of a SystemMessage.

The key point is that SystemPromptTemplate inherits the Builder from PromptTemplate.

I can think of two ways to address this problem, but in my opinion, neither is perfect:

1. Add a Builder inner class in SystemPromptTemplate that extends the Builder class of PromptTemplate, and override all of its methods. However, this approach would require updating all subclasses' Builder classes whenever any new properties are added to PromptTemplate in the future.

2. Make the Builder class of PromptTemplate generic and use the “Simulated Self-Type Idiom” so that each subclass can return its own Builder type. However, this would involve significant changes to the current Builder class of PromptTemplate, and all existing code that uses it would also need to support generics.

Currently, I have adopted the first approach because it has a smaller impact scope compared to the second one.

This PR includes the following changes:

1. Adjusted the visibility of some fields and methods in the Builder of PromptTemplate to support inheritance in the Builder of SystemPromptTemplate.
2. Added a Builder for SystemPromptTemplate.
3. Added relevant unit tests.

Fixes #3526