[FEATURE] Accept template from content objects FLUIDTEMPLATE

Add possibility to use content objects as Fluid
template source.

This adds a new property "template" to the FLUIDTEMPLATE
cObj to be in line with the TEMPLATE cObj

Resolves: #39767
Releases: 6.1

Change-Id: Iacd291f503ab88fb530f3b439e26cb755d17dee2
Reviewed-on: https://review.typo3.org/16241
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn