fleet: require a minimum version of the peer at the handshake [CI]

MoShiro__MinAgentVersion makes the backend refuse agents below it at hello (fleet_version_required); fleet.min_server_version or MOSHIRO_MIN_SERVER_VERSION makes the agent drop a backend below it. Either side waits five seconds and shakes hands again, because the other one may be updating.

backend/fleet/src/Fleet.Server.Abstractions/FleetServerOptions.cs

@@ -22,3 +22,12 @@ public sealed class FleetServerOptions
     /// <summary>节点违反协议（方法没有声明、方向、类型或角色不对）的次数上限；超过就断开这条连接。</summary>
     public int MaxViolations { get; set; } = 20;
 }
+
+/// <summary>握手时的版本要求：服务端的版本（告诉节点），以及节点至少要是哪个版本。每次握手都重新取，所以改配置不用重启。</summary>
+public interface IFleetVersionPolicy
+{
+    string? ServerVersion { get; }
+
+    /// <summary>空表示不要求。</summary>
+    string? MinNodeVersion { get; }
+}