Enable New Code based on "reference branch" with a scanner parameter

We introduce a new parameter used on the scanner side, 'sonar.newCode.referenceBranch'.
The parameter affects the scanner in the following way:

Assumes that the New Code Period is set to "reference branch", using the given branch as the reference.
Skips fetching the New Code Period setting from the server
Rest of the behavior remains unchanged, including getting diff from the SCM.