GH-4251 Add the capability to parse required variables from templates to TemplateRenderer

[sunyuhan1998]

As mentioned in the issue, TemplateRenderer currently only supports rendering templates. The ability to also parse and extract the required variables from a given template would be highly beneficial, especially for proactively validating template correctness. This PR implements that capability.

Specifically, it:

1. Adds a getRequiredVariables method to TemplateRenderer to extract the variable names required by the template;

2. Includes corresponding unit tests to ensure correctness and stability.

Fixes #4251

[ericbottard]

Hi @sunyuhan1998,
thanks for your contribution!

Could you please rebase against main and also add the following logic: not all implementations may support retrieving variable names. As a consequence, could you make the new method throw NotSupportedException and make it default

[sunyuhan1998]

Could you please rebase against main and also add the following logic: not all implementations may support retrieving variable names. As a consequence, could you make the new method throw NotSupportedException and make it default

Thank you very much for your review. I agree with your comments. I have merged the code from the main branch and adjusted the getRequiredVariables method as you suggested. However, I couldn't seem to find the NotSupportedException class in the project, so I used OperationNotSupportedException instead. Do you think this is acceptable? Could you please review it once again? Thank you so much!