Add step-up and step-down hooks for the IndexBuildsCoordinator.

There's a field on the mongod implementation that indicates primary/secondary state https://github.com/mongodb/mongo/blob/5eca4a77da863bd4e68bf4eb7c2d0c920982f8b9/src/mongo/db/index_builds_coordinator_mongod.h#L137

And there are interface functions for setting primary and secondary, which look like they've been implemented in the mongod and embedded already. https://github.com/mongodb/mongo/blob/5eca4a77da863bd4e68bf4eb7c2d0c920982f8b9/src/mongo/db/index_builds_coordinator.h#L197-L198

We'll need to call the state change functions in here https://github.com/mongodb/mongo/blob/4d09b2e0a605aefd7adefda28e01e309bbf30883/src/mongo/db/repl/replication_coordinator_external_state_impl.cpp#L483 and it looks like stepdown hooks go in here maybe https://github.com/mongodb/mongo/blob/4d09b2e0a605aefd7adefda28e01e309bbf30883/src/mongo/db/repl/replication_coordinator_impl.cpp#L2800 – check in with someone from repl to find out their preferences for new hook additions.