Issue authenticating with ACR

[juliusl]

Hello, I was having issues authenticating with ACR. I looked at the code and noticed it uses the basic auth flow w/ GET request. For ACR, we use the POST refresh token method -- Is there a way to use alternative auth modes?

https://github.com/containerd/overlaybd/blob/938be8b2f19735c229d3b49f40cea5936b61965c/src/overlaybd/registryfs/registryfs.cpp#L295-L302

Example curl of what I mean

                curl -v -X POST -H "Content-Type: application/x-www-form-urlencoded" -d \
                "grant_type=refresh_token&service=$registry&scope=$scope&refresh_token=$acr_refresh_token" \
                https://$registry/oauth2/token

[liulanzheng]

abstract an out auth api and dynamic link the implementation？ @Coldwings any suggestions?