[FEATURE REQ] ACR: Optimize auth flow

[lmolkova]

Library name

Azure.Containers.ContainerRegistry

Please describe the feature.

ACR uses oauth2 and gets refresh token and access token.
A refresh token is obtained with AAD token and cached in memory. It's the same for the repository.

We get access token on each request using a refresh token. The access token is scoped to specific actions and repository. It's valid (currently) for about an hour.

It's safe and doable to cache access token (either per action + client instance or bluntly once) and only refresh it on 401.

this is not blocking GA.