Suppress gRPC build warning in SPM using command plugins

Recent updates on SPM SP-0332 allows post-install / pre-build commands to run similar to Cocoapod. This allows us to suppress warnings in gRPC build when downstream projects install gRPC.

This feature is available under

SPM 5.6 (https://github.com/apple/swift-package-manager/blob/d5b1b75bb77e6d16128cfd90f88bc5c9db92b38c/Documentation/ReleaseNotes/5.6.md)
Xcode 13.3+ Apple Doc https://developer.apple.com/documentation/swift_packages/plugincommandintent

grpc / grpc-ios

Suppress gRPC build warning in SPM using command plugins #95