Introduce TemplateRenderer for prompt templating

- Introduce new TemplateRenderer API providing the logic for rendering an input template.
- Update the PromptTemplate API to accept a TemplateRenderer object at construction time.
- Move ST logic to StTemplateRenderer implementation, used by default in PromptTemplate. Additionally, make start and end delimiter character configurable.

Relates to gh-2655

Signed-off-by: Thomas Vitale <ThomasVitale@users.noreply.github.com>

pom.xml

@@ -33,6 +33,7 @@
 		<module>spring-ai-docs</module>
 		<module>spring-ai-bom</module>
 		<module>spring-ai-commons</module>
+		<module>spring-ai-template-st</module>
 		<module>spring-ai-client-chat</module>
 		<module>spring-ai-model</module>
 		<module>spring-ai-test</module>