Azure can only authenticate with client_id through environment variables

[DylanGuedes]

Hello,
I noticed that Thanos cannot authenticate with Azure when using the UserAssignedID as it doesn't allow you to set a client secret and a tenant id.

I could make it to work through environment variables, which is fine for most use cases, but for the use case where you want to interact with multiple Azure buckets, this makes it impossible. The environment variables I used are AZURE_TENANT_ID, AZURE_CLIENT_ID and AZURE_CLIENT_SECRET.

So I'm creating this ticket to track/request the possibility of setting client_secret and tenant_id when using UserAssigendID.

[DylanGuedes]

PR #157 tackles this