[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.