VPPAPIGEN: Add union and enum support and IP4/IP6 address type.

VPPAPIGEN: Add union and enum support and IP4/IP6 address type.

Note: The Python, Java and C/C++ bindings must be updated before ip/ip_types.api can be used.

ip_types.api:

typedef ip4_address {
  u8 address[4];
};

typedef ip6_address {
  u8 address[16];
};

enum address_family {
  ADDRESS_IP4 = 0,
  ADDRESS_IP6,
};

union address_union {
  vl_api_ip4_address_t ip4;
  vl_api_ip6_address_t ip6;
};

typedef address {
  vl_api_address_family_t af;
  vl_api_address_union_t un;
};

Change-Id: I22f67092f24db5bd650a03c6f446a84cd9fd1074
Signed-off-by: Ole Troan <ot@cisco.com>