Allow simple response builder prompt change (#505)

.changeset/gorgeous-rocks-grow.md

+---
+"llamaindex": patch
+---
+
+easier prompt customization for SimpleResponseBuilder

packages/core/src/synthesizers/builders.ts

@@ -33,11 +33,11 @@ enum ResponseMode {
  */
 export class SimpleResponseBuilder implements ResponseBuilder {
   llm: LLM;
-  textQATemplate: SimplePrompt;
+  textQATemplate: TextQaPrompt;
 
-  constructor(serviceContext: ServiceContext) {
+  constructor(serviceContext: ServiceContext, textQATemplate?: TextQaPrompt) {
     this.llm = serviceContext.llm;
-    this.textQATemplate = defaultTextQaPrompt;
+    this.textQATemplate = textQATemplate ?? defaultTextQaPrompt;
   }
 
   getResponse(