Run with SpringTemplateEngine

[vandykej17]

Is your feature request related to a problem? Please describe.
This is kind of a bug, but also a feature. We are seeing the thymeleaf tag engine is not doing normal "spring things" like CSRF tags. I tracked this down to the FF4J servlet explicitly calling out to use the TemplateEngine instead of SpringTemplateEngine. I understand not everyone using ff4j-web is using spring, so this is a feeler on how we can make this work for both Spring users and non-Spring users.

https://github.com/ff4j/ff4j/blob/main/ff4j-web/src/main/java/org/ff4j/web/FF4jServlet.java#L180

Describe the solution you'd like
I'd like the ff4j spring boot config to use the SpringTemplateEngine so it auto-injects the CSRF tokens. I assume this would possibly require a change in the ff4j-web project to allow such customization. With that said, it doesn't seem too far-fetched to allow a runtime config for which template engine to use.

Describe alternatives you've considered
I have a hacky workaround right now doing a sed command in my Dockerfile and adding the CSRF hidden inputs in.

Thanks!

[vandykej17]

Forgot to put this above... This is the feedback I received from the ff4j-web project.
ff4j/ff4j#687