main/cdr: Allow modules to modify CDR fields before dispatching them

This patch adds the functions

	ast_cdr_modifier_register()
	ast_cdr_modifier_unregister()

That work much like ast_cdr_register() and ast_cdr_unregister().

Modules registered will be given a chance to modify (or to do whatever
they want) CDR fields just before they are passed to registered engines.

Thus, for instance, if a module change the "userfield" field of a CDR,
the modified value will be passed to every registered CDR backend for
logging.

ASTERISK-25479 #close

Change-Id: If11d8fd19ef89b1a66ecacf1201e10fcf86ccd56