External il2cpp stubs (#3330)

* Fix Windows IL2CPP compilation.  This does not address the issues with training when IL2CPP is enabled.
Co-authored-by: Jérémy Graziani <AcelisWeaven@users.noreply.github.com>

com.unity.ml-agents/Plugins/IL2CPP.DL.Stubs.c

+// These stubs fix an issue compiling GRPC on Windows with IL2CPP.
+// For the moment, only Inference works. (training doesn't)
+
+void * dlopen(const char *filename, int flags) {
+    return 0;
+}
+
+void * dlsym(void *handle, const char *symbol) {
+    return 0;
+}

com.unity.ml-agents/Plugins/IL2CPP.DL.Stubs.c.meta

+fileFormatVersion: 2
+guid: 3509a8908cf600c4f914a0705123a363
+PluginImporter:
+  externalObjects: {}
+  serializedVersion: 2
+  iconMap: {}
+  executionOrder: {}
+  defineConstraints: []
+  isPreloaded: 1
+  isOverridable: 0
+  isExplicitlyReferenced: 0
+  validateReferences: 1
+  platformData:
+  - first:
+      '': Any
+    second:
+      enabled: 0
+      settings:
+        Exclude Editor: 1
+        Exclude Linux: 1
+        Exclude Linux64: 1
+        Exclude LinuxUniversal: 1
+        Exclude OSXUniversal: 1
+        Exclude Win: 0
+        Exclude Win64: 0
+  - first:
+      Any: 
+    second:
+      enabled: 0
+      settings: {}
+  - first:
+      Editor: Editor
+    second:
+      enabled: 0
+      settings:
+        CPU: AnyCPU
+        DefaultValueInitialized: true
+        OS: AnyOS
+  - first:
+      Facebook: Win
+    second:
+      enabled: 0
+      settings:
+        CPU: AnyCPU
+  - first:
+      Facebook: Win64
+    second:
+      enabled: 0
+      settings:
+        CPU: AnyCPU
+  - first:
+      Standalone: Linux
+    second:
+      enabled: 0
+      settings:
+        CPU: None
+  - first:
+      Standalone: Linux64
+    second:
+      enabled: 0
+      settings:
+        CPU: None
+  - first:
+      Standalone: LinuxUniversal
+    second:
+      enabled: 0
+      settings:
+        CPU: None
+  - first:
+      Standalone: OSXUniversal
+    second:
+      enabled: 0
+      settings:
+        CPU: None
+  - first:
+      Standalone: Win
+    second:
+      enabled: 1
+      settings:
+        CPU: AnyCPU
+  - first:
+      Standalone: Win64
+    second:
+      enabled: 1
+      settings:
+        CPU: AnyCPU
+  userData: 
+  assetBundleName: 
+  assetBundleVariant: