feat: exclude nodes with zero balance from node_groups

[noot]

• get and store balance in DiscoveryNode in discovery::chainsync::sync_single_node()
• in the orchestrator's sync_single_node, handle the case where the node has zero balance by setting status to NodeStatus::LowBalance (todo: is there a non-zero balance we can set this to?)
• in NodeGroupsPlugin::handle_status_change, dissolve the group if the node has low balance (not sure if this is the intended behaviour)

closes #408