[pigeon] Reland: Add an initial example app - #3832

[pigeon] Reland: Add an initial example app#3832

auto-submit[bot] merged 2 commits into

flutter:mainflutter/packages:mainfrom

stuartmorgan-g:pigeon-example-app-relandstuartmorgan-g/packages:pigeon-example-app-relandCopy head branch name to clipboard