PAPI: Add support for format/unformat functions.

PAPI: Add support for format/unformat functions.

With the introduction of new types, like vl_api_address_t
it is now possible to call a message using one of those
functions with a string representation. E.g. for an IP address

ip_add_address(address="1.1.1.1/24")

The language wrapper will automatically convert the string
into the vl_api_address_t representation. Currently
the caller must do the reverse conversion from the returned
named tuple with the unformat function.
rv = get_address_on_interface(sw_if_index=1)
print(VPPFormat.unformat(rv.address))

Change-Id: Ic872b4560b2f4836255bd5260289bfa38c75bc5d
Signed-off-by: Ole Troan <ot@cisco.com>