Allow clientId hint when using Managed Identity

[ArvidMartensRenson]

Azure supports to use multiple user assigned managed identities each having its own permissions on resources. This PR makes it possible to assign a specific identity to a specific grafana azure data source.

[CLAassistant]

All committers have signed the CLA.

[ArvidMartensRenson]

@asimpson @bossinc @aangelisc @alyssabull
Do you have a timeline on when this is getting reviewed and released? We are eager to start implementing the necessary changes in the appropriate datasources.

[aangelisc]

Hi @ArvidMartensRenson, we are currently working through some other higher-priority work. As soon as we get around to this we will review and either merge or discuss the implementation. Thanks!