[RPC service] Add failover support

[Gudahtt]

Add failover support to the RPC service. The service should accept additional URLs to use in case the circuit is open on the primary RPC URL.

Requests should automatically be sent to the failover URL in case the primary URL is "down" (i.e. if the circuit is open). However we'd want to continue sending each request through the policy for the primary, so that it can detect when that service recovers.

For now we only need to support a single failover RPC URL, but it might be useful to support a list right from the start if it's not too much of a detour.