Support client certificates for App Registration authentication

[kostrse]

Currently App Registration authentication type only supports client secrets.
Azure AD also supports client certificates but they are not supported by Grafana.

The goal is to support client certificates as well:

Tasks

Beta Give feedback

1. Support client certificates for App Registration grafana-azure-sdk-go#87
   azure-sdk enhancement priority/med +3
2. App Registration with client certificate in Azure crednetials UI grafana-azure-sdk-react#117
   azure-sdk-react enhancement priority/med +3

[hcoona]

And I further request if it was possible to use the certificate from specific Azure Key Vault.