Fix issue which prevented training when code stripping was enabled (#2662)

UnitySDK/Assets/ML-Agents/Plugins/ProtoBuffer/link.xml

+<?xml version="1.0" encoding="utf-8"?>
+<linker>
+<!--
+Prevents Managed code stripping from removing code necessary for training.
+You may wish to disable this preservation (that is, re-enable code stripping) to
+reduce code size if you no longer need training. 
+See also https://docs.unity3d.com/Manual/ManagedCodeStripping.html
+-->
+    <assembly fullname="Grpc.Core" preserve="all"/>
+</linker>

UnitySDK/Assets/ML-Agents/Plugins/ProtoBuffer/link.xml.meta

+fileFormatVersion: 2
+guid: f94355fa6eab94c2d8529747b92ca3e1
+TextScriptImporter:
+  externalObjects: {}
+  userData: 
+  assetBundleName: 
+  assetBundleVariant: