Link to identity

[ahmed-ebaid]

This PR closes # 978

The changes in this PR are as follows:

• Identity: link_to_identity - configures applications to use a UserAssignedIdentity of LinkedResource type of Unmanaged

I have read the contributing guidelines and have completed the following:

• [x] Tested my code end-to-end against a live Azure subscription.
• [x] Updated the documentation in the docs folder for the affected changes.
• [x] Written unit tests against the modified code that I have made.
• [x] Updated the release notes with a new entry for this PR.
• [x] Checked the coding standards outlined in the contributions guide and ensured my code adheres to them.

If I haven't completed any of the tasks above, I include the reasons why here:

Below is a minimal example configuration that includes the new features, which can be used to deploy to Azure:

let resourceId =
    ResourceId.create (ManagedIdentity.userAssignedIdentities, ResourceName "user", "resourceGroup")

let userAssignedIdentity =  resourceId |> UserAssignedIdentity

let managedIdentity =
    { ManagedIdentity.Empty with
        UserAssigned = [ userAssignedIdentity ]
    }

let group =
    containerGroup {
        add_instances [ nginx ]
         link_to_identity resourceId
         add_managed_identity_registry_credentials [ registry "my-registry.azurecr.io" "user" managedIdentity ]
    }
    |> asAzureResource