[Az.ManagedServices] Updating existing Managed Services Definition creates a new definition

## Resources

* Documentation - https://docs.microsoft.com/en-us/powershell/module/az.managedservices/New-AzManagedServicesDefinition?view=azps-3.6.1#description
* Module (with version I used): https://www.powershellgallery.com/packages/Az.ManagedServices/1.0.2

## Description / Reproduction

According to linked documentation, you can use "New-AzManagedServicesDefinition" to create or update existing registration definition.

From what I can see of examples, the following should be all parameters needed to add a new authorization to an existing Manages Services Definition, or am I wrong / not understanding its' use?

```powershell
$null = New-AzManagedServicesDefinition -Name $ExistingAzManagedServicesDefinitions.'Properties'.'Name' `
-ManagedByTenantId $ExistingAzManagedServicesDefinitions.'Properties'.'ManagedByTenantId' `
-PrincipalId $Authorization.'Key' -RoleDefinitionId $Authorization.'Value'
```

Where:

* $ExistingAzManagedServicesDefinitions is an existing Managed Services Definition found with "Get-AzManagedServicesDefinition".
* $Authorization is a hashtable containing a single Azure AD group from provider tenant (Key) and RBAC role id (Value) for "Reader".

## Expected Result

A new authorization is added to the existing Managed Services Definition.

## Actual result

The command runs successfully, but on managed tenant a new Managed Services Definition appears with the exact same name as the existing one, and no assignments.

## Questions

* How am I supposed to update: add or remove (I haven't dared trying this yet) authorizations to existing Managed Services Definitions with the "New-AzManagedServicesDefinition" and "Remove-AzManagedServicesDefinition" commands, as the documentation states is possible?
* Given the original issue here: How can I create a Managed Services Definition with multiple authorizations when "New-AzManagedServicesDefinition" only takes a single authorization as input?

*This module for Managed Services seems kind of useless without having said abilities tbh.*