lib: enhance pbr_rule structure for zapi encode and for common usage

lib: enhance pbr_rule structure for zapi encode and for common usage

The pbr_rule structure is derived from zebra_pbr_rule, and is
defined, so that a zclient will be able to encode the zebra_pbr_rule to
send ADD_RULE or DEL_RULE command. Also, the same structure can be used
by other daemons to derive a structure ( this will be the case for
zebra_pbr_rule).
Adding to this, an encoding function is defined, and will be used by
remote daemon to encode that message.
Those definitions are moved in new file pbr.h file.

Signed-off-by: Philippe Guibert <philippe.guibert@6wind.com>