Allow optional custom STUN servers

[tbraun96]

Allow each node to choose which STUN servers to communicate with when attempting to find their NAT type. 3 must be specified. This setting should be present inside the NodeBuilder struct in citadel_sdk, and, should propagate all the way to the library citadel_wire where the STUN servers are used. This will require editing many function signatures, but, will be a great first issue to solve as it will walk one through the execution flow of the program.