app_if: Adds conditional branch applications

Adds the If, ElseIf, Else, ExitIf, and EndIf
applications for conditional execution
of a block of dialplan, similar to the While,
EndWhile, and ExitWhile applications. The
appropriate branch is executed at most once
if available and may be broken out of while
inside.

ASTERISK-29497

Change-Id: I3aa3bd35a5add82465c6ee9bd86b64601f0e1f49

doc/CHANGES-staging/app_if.txt

@@ -0,0 +1,4 @@
+Subject: app_if
+
+Adds the If, ElseIf, Else, EndIf, and ExitIf applications
+for conditional execution of a block of code.