Allow repeated commands to same connection in pool

Motivation:

Some Redis commands are very connection specific that have impacts on future access that makes it difficult in the current
checkout-use-return cycle that `RedisConnectionPool` uses.

Developers need a way to borrow a specific connection, chain several commands together, and then return the connection to the pool.

Modifications:

- Add: `leaseConnection` method to `RedisConnectionPool` which provides a connection from the pool and returns it after a provided closure's ELF resolves
- Add: `allowSubscriptions` property to `RedisConnection` for controlling the ability to make PubSub subscriptions
- Add: `RedisClientError.pubsubNotAllowed` case for when `RedisConnection.allowSubscriptions` is set to `false` and a subscription was still attempted

Result:

Developers should now have an "escape hatch" with `RedisConnectionPool` to do limited exclusive chains of operations on a specific connection.

Sources/RediStackTestUtils/RedisConnectionPoolIntegrationTestCase.swift

@@ -75,14 +75,15 @@ open class RedisConnectionPoolIntegrationTestCase: XCTestCase {
     }
     
     public func makeNewPool(
-        connectionRetryTimeout: TimeAmount? = .seconds(5)
+        connectionRetryTimeout: TimeAmount? = .seconds(5),
+        minimumConnectionCount: Int = 0
     ) throws -> RedisConnectionPool {
         let address = try SocketAddress.makeAddressResolvingHost(self.redisHostname, port: self.redisPort)
         let pool = RedisConnectionPool(
             serverConnectionAddresses: [address],
             loop: self.eventLoopGroup.next(),
             maximumConnectionCount: .maximumActiveConnections(4),
-            minimumConnectionCount: 0,
+            minimumConnectionCount: minimumConnectionCount,
             connectionPassword: self.redisPassword,
             connectionRetryTimeout: connectionRetryTimeout
         )