fix(internal): Lower constraints when retrieving _internal

`RUMMonitorProtocol._internal` required that self be the type `Monitor` before creating the DatadogInternalInterface. This was overspecialization and made it hard to mock internal calls. By casting only to `RUMCommandSubscriber` we can more easily create mocks for testing.

DatadogRUM/Sources/RUMMonitorProtocol+Internal.swift

@@ -12,7 +12,7 @@ public extension RUMMonitorProtocol {
     /// Grants access to an internal interface utilized only by Datadog cross-platform SDKs.
     /// **It is not meant for public use** and it might change without prior notice.
     var _internal: DatadogInternalInterface? {
-        guard let monitor = self as? Monitor else {
+        guard let monitor = self as? RUMCommandSubscriber else {
             return nil
         }
         return DatadogInternalInterface(monitor: monitor)