Builtin Functions for Groovy and Template

[predic8]

Continue work on PR:

#2322

Goal:

Be able to use the same built in functions in groovy templates as in SpEL expressions.

      - template:
          src: |
            {
              "sub": "${fn.user()}",
            }

We are using the complied StreamingTemplateEngine from Groovy that is not dynamic as the other Template engines. So we cannot just implement it without fn : ${user()}!

Steps:

1. Have a look at the changes from:

#2322

2. Check out branch yaml-tutorial-2. The commit from above is committed there
3. Implement the TODOs of the classes from the commit

[predic8]

@rrayst Plz take a look if that issue makes sense