p2p/discover: add helper methods to UDPv5 (#20918)

This adds two new methods to UDPv5, AllNodes and LocalNode.

AllNodes returns all the nodes stored in the local table; this is
useful for the purposes of metrics collection and also debugging any
potential issues with other discovery v5 implementations.

LocalNode returns the local node object. The reason for exposing this
is so that users can modify and set/delete new key-value entries in
the local record.