feat: add optimistic locking to Firestore sample app (#188)#4311

mehdi0933 wants to merge 2 commits intoGoogleCloudPlatform:mainGoogleCloudPlatform/spring-cloud-gcp:mainfrom

mehdi0933:fix-firestore-lockingmehdi0933/spring-cloud-gcp:fix-firestore-lockingCopy head branch name to clipboard