make TiCDC support managed identity for Azure

[wlwilliamx]

Today things like blob store access on azure is controlled by authentication via fixed azure keys/secrets as mentioned in: Grant Permissions to Remote Storage

This approach is less secure than managed identities which is managed and provisioned by azure gated by the vm identity and we would like to switch to using that: Managed identities for Azure resources - Managed identities for Azure resources

Currently this not appear to be supported and we would like to make a feature request to have this supported.

Notably instead of using a k8s secret (that is fixed and requires us to manage/rotate), I believe this feature would require binaries (like tikv) to query the local endpoint to obtain the credentials

e.g. How managed identities for Azure resources work with Azure virtual machines - Managed identities for Azure resources

curl 'http://169.254.169.254/metadata/identity/oauth2/token?api-version=2018-02-01&resource=https%3A%2F%2Fstorage.azure.com%2F' -H Metadata:true