[network-controller] Add failover support

[Gudahtt]

Add failover support to the NetworkController. This depends upon the service supporting failovers first (#4992), and upon integration of the RPC service (#4991).

We'll need to keep track of a list of failover RPC URLs for each RPC endpoint that we want to have failover support. It probably makes sense to add this failover list to the InfuraRpcEndpoint type.

[mcmire]

After talking with @Gudahtt about this, we will need some way for consumers to listen for when certain events occur, so that it is possible to report when a failover occurs. I might need to reach out to the SRE team to figure out what else we would need. But at the very least we need to add callback options to NetworkController.