`TemplateRenderer` should support retrieving required variables from the template.

[sunyuhan1998]

Expected Behavior

Currently, TemplateRenderer only supports rendering templates. It would be highly beneficial if it could also support parsing the required variables from a given template, as this would greatly assist us in proactively validating templates. For example, in org.springframework.ai.rag.util.PromptAssert#templateHasRequiredPlaceholders, the current validation relies solely on contains, which is clearly prone to oversights.

Current Behavior

Currently, TemplateRenderer only supports rendering templates.