Provide a pre-populated list of nodes when setting API/BE nodes

Currently we only support textual entry for hive and block explorer nodes configuration.

I think it would add value to have a prepopulated list of available nodes (https://beacon.peakd.com has such a list), so when a user is attempting to change a node, he can actually select an existing list, or enter a custom one manually. So both selection and custom entry should be supported.