Add support for authenticating against Azure Container Registry

[ThorstenHans]

As described in #262 Azure Container Registry returns the access token using the access_token field instead of the token field. With this commit, the token could be fetched from both fields

[ThorstenHans]

Upon further testing I discovered that Docker Hub returns both fields token and access_token in the response. I know that the name of the added struct (AzureBearerAuth) is not optimal. But using an intermediate struct allowed me to successfully authenticate against both Docker Hub and ACR.

[vrutkovs]

cc @PratikMahajan

[ThorstenHans]

Renamed the struct from AzureBearerAuth to MultiTokenBearerAuth which is a bit more meaningful