Simple YAML fn.concat intrinic function proposal (string concat)

The following link goes into details, describing a proposition for the addition of a new intrinsic function 'fn.concat' to the specification, to enable string concatenation in operations inputs/outputs/etc..

https://docs.google.com/document/d/11rP626tWJrdPE9Jdh5OsT4Y5lVzjbOJcbUVVNT9m2rk/edit?usp=sharing

This proposition extends and formalizez the idea expressed in a the JIRA issue https://issues.oasis-open.org/browse/TOSCA-212 opened by Luc Boutier.