feat: support multiple ethereum grpc endpoints

[toteki]

Another possible way to do this:

• The one-off commands in bridge.go should be fine iterating their way through endpoints until they find a good one
• Possible improvement discussed in comments: remembering the last successfully dialed and always trying it first
• Still have some questions about the one in orchestrator.go, especially (if it's being reused over time) when to try to redial

[toteki]

Superseded by #215