Initial support for Azure MSI authentication

[nfx]

Authenticating with Azure MSI

Since v0.3.8, it's possible to leverage Azure Managed Service Identity authentication, which is using the same environment variables as azurerm provider. Both SystemAssigned and UserAssigned identities work, as long as they have Contributor role on subscription level and created the workspace resource, or directly added to workspace through databricks_service_principal.

provider "databricks" {
  host = data.azurerm_databricks_workspace.this.workspace_url
  
  # ARM_USE_MSI environment variable is recommended
  azure_use_msi = true 
}

[codecov]

Codecov Report

Merging #743 (a1ef8d6) into master (94cd29c) will decrease coverage by 0.06%.
The diff coverage is 67.50%.

@@            Coverage Diff             @@
##           master     #743      +/-   ##
==========================================
- Coverage   82.98%   82.92%   -0.07%     
==========================================
  Files          92       92              
  Lines        8179     8191      +12     
==========================================
+ Hits         6787     6792       +5     
- Misses        891      898       +7     
  Partials      501      501              

Impacted Files	Coverage Δ
access/resource_secret_scope.go	87.50% <0.00%> (ø)
identity/resource_service_principal.go	84.41% <ø> (ø)
common/azure_auth.go	72.94% <47.82%> (-2.97%)	⬇️
common/azure_cli_auth.go	95.00% <100.00%> (+2.81%)	⬆️
common/client.go	80.60% <100.00%> (-0.12%)	⬇️
common/gcp.go	83.33% <100.00%> (ø)
common/http.go	85.46% <100.00%> (ø)
provider/provider.go	94.44% <100.00%> (+0.04%)	⬆️
sqlanalytics/resource_widget.go	86.13% <0.00%> (+0.72%)	⬆️

[alexott]

Code looks good, we need docs for it.

[nfx]

reviewed again and looks fine