Depend on grpc-swift main during development

Author: gjcairo

Package.swift

@@ -39,7 +39,7 @@ let products: [Product] = [
 let dependencies: [Package.Dependency] = [
   .package(
     url: "https://github.com/grpc/grpc-swift.git",
-    exact: "2.0.0-beta.2"
+    branch: "main"
   ),
   .package(
     url: "https://github.com/grpc/grpc-swift-protobuf.git",