add RADIUS support for IP address assignment via a network manager

[spencersevilla]

Previously, the SPGW dynamically assigned IP addresses to UEs from a pool; this pool is configured in package/epc/spgw.conf as IPV4_LIST. IP address assignment took place every time a UE joined the network. This happened in pgw_lite_paa.c; see pgw_get_free_ipv4_paa_address() for assignment and pgw_release_free_ipv4_paa_address() for release.

0c7e9a7 changed this by adding new functionality that replaced this dynamic assignment with a database lookup. The new IP address assignment function is spgw_get_ip_from_imsi(). The changes in src/sgw/pgw_ue_ip_address_alloc.c show how we replaced the old functionality with the new functionality.

The new goal/design/architecture is to dynamically move between these two options and remove ALL database calls from the SPGW. Tasks are as follows:

1. Add a RADIUS library to the SPGW, and add any needed RADIUS configuration options to spgw.conf.
2. On start-time (main() is in src/oai_sgw/oai_sgw.c) the codebase should attempt to use these options to talk to a network manager such as haulage.
3. Add some logic in pgw_ue_ip_address_alloc.c: If we have a RADIUS connection, use it to query the network manager to ask what the IP address should be for this IMSI. If we could not establish a RADIUS connection, fallback to dynamic assignment out of the IPV4_LIST pool.