[Cosmos] Add optimistic concurrency samples (etag/ifmatch)

[simorenoh]

It was recently pointed out to me that we were missing samples around the use of etags and if-match (optimistic concurrency) for our SDK - we do have tests, but these samples were missing. For more on optimistic concurrency: https://learn.microsoft.com/azure/cosmos-db/nosql/database-transactions-optimistic-concurrency

[azure-sdk]

API change check

API changes are not detected in this pull request.