test: raft: randomized_nemesis_test: network: `add_grudge`, `remove_grudge` functions
test: raft: randomized_nemesis_test: network: `add_grudge`, `remove_grudge` functions

Extend the interface of `network` to allow introducing and removing "grudges"
which prevent the delivery of messages from one given server to another
(when the time comes to deliver a message but there's a grudge, the
message is dropped).

scylladb/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla