Allow reuse of a Redis ConnectionMultiplexer instance between RedisCa…

…che and RedisBackplane (#177)

Added a factory method (ConnectionMultiplexerFactory) to RedisBackplaneOptions to allow passing a ConnectionMultiplexer instance.

src/ZiggyCreatures.FusionCache.Backplane.StackExchangeRedis/RedisBackplaneOptions.cs

@@ -1,4 +1,6 @@
-using Microsoft.Extensions.Options;
+using System;
+using System.Threading.Tasks;
+using Microsoft.Extensions.Options;
 using StackExchange.Redis;
 
 namespace ZiggyCreatures.Caching.Fusion.Backplane.StackExchangeRedis;
@@ -20,6 +22,11 @@ public class RedisBackplaneOptions
 	/// </summary>
 	public ConfigurationOptions? ConfigurationOptions { get; set; }
 
+	/// <summary>
+	/// Gets or sets a delegate to create the ConnectionMultiplexer instance.
+	/// </summary>
+    public Func<Task<IConnectionMultiplexer>>? ConnectionMultiplexerFactory { get; set; }
+
 	/// <summary>
 	/// The configuration used to connect to Redis.
 	/// </summary>

src/ZiggyCreatures.FusionCache.Backplane.StackExchangeRedis/RedisBackplane_Sync.cs

@@ -20,7 +20,10 @@ private void EnsureConnection(CancellationToken token = default)
 			if (_connection is not null)
 				return;
 
-			_connection = ConnectionMultiplexer.Connect(GetConfigurationOptions());
+			_connection = _options.ConnectionMultiplexerFactory is null 
+				? ConnectionMultiplexer.Connect(GetConfigurationOptions())
+				: _options.ConnectionMultiplexerFactory().GetAwaiter().GetResult();
+
 			if (_connection is not null)
 			{
 				_connection.ConnectionRestored += OnReconnect;