fix: Replace `roundrobin` flag with `AgentSelectionStrategy.RoundRobin` strategy

[jopemachine]

Checklist: (if applicable)

• [x] Milestone metadata specifying the target backport version
• [x] Mention to the original issue

Follow up PR of #1405. This PR performs overall revamps related AgentSelectionStrategy things.

Details:

• Replace the roundrobin flag with AgentSelectionStrategy.RoundRobin strategy
• Simplify the roundrobin agent selection logic when joined agent list changes.
• Move agent_selection_resource_priority into AbstractScheduler to avoid passing it all related functions each time.
• Fix wrong type declaration (Replace KernelInfo with KernelRow of assign_agent_for_kernel)
• Abstract agent selection logic into AbstractScheduler.

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How to test

To test the RoundRobin strategy,

• To use the RoundRobin policy for agent_selection_strategy, change the agent_selection_strategy in scaling_groups.scheduler_opts to roundrobin.
• Update the redis address in etcd by ./backend.ai mgr etcd put config/redis/addr <manager_addr>:8111
• Update the rpc-listen-addr, id of the agent section in each agent's agent.toml.
• Checks if all agents are properly connected to the manager in the Backend.AI web UI.
• Create sessions to verify that sessions are evenly distributed among the agents and that the next_index for round_robin state is correctly updated in etcd.