Support of template import in the freemarker engine

[M3lkior]

The aims are to support the freemarker macro feature in order to use the freemarker inline script like

<#import "simple_macro.ftl" as myMacro><@myMacro.p >Hello world!</@myMacro.p>

with simple_macro.ftl deploying into a templates folder in the camunda classpath

<#macro p><p><#nested></p></#macro>

Using the external resource must also work :

Using freemarker/test.ftl (in a different folder than templates) with the following test.ftl content :

<#import "simple_macro.ftl" as simple>

<@simple.p>it works</@simple.p>

A related PR (tested with Camunda 7.20 is almost ready)