factor grpc extension out to ext-grpc, keep library for now, even though unused

config/ext.json

@@ -232,7 +232,7 @@
             "BSD": "wip"
         },
         "type": "external",
-        "source": "grpc",
+        "source": "ext-grpc",
         "arg-type-unix": "enable-path",
         "cpp-extension": true,
         "lib-depends": [

config/source.json

@@ -151,6 +151,18 @@
             "path": "LICENSE"
         }
     },
+    "ext-grpc": {
+        "type": "url",
+        "url": "https://pecl.php.net/get/grpc",
+        "path": "php-src/ext/grpc",
+        "filename": "grpc.tgz",
+        "license": {
+            "type": "file",
+            "path": [
+                "LICENSE"
+            ]
+        }
+    },
     "ext-imagick": {
         "type": "url",
         "url": "https://pecl.php.net/get/imagick",