Add name validation for azurerm_role_assignment

[neil-yechenwei]

This PR is the fix of the issue #5543

After investigated, I found the root cause is the property "name" in azurerm_role_assignment the user provided includes character "/" and the client http request url would be appended "AtlasPeeringRoleAssignment/xxxx/xxxx/xxxx". So this request url cannot be found from server side.
After checked, the property "name" should be guid according by the doc(https://docs.microsoft.com/en-us/rest/api/authorization/roleassignments/create).

Fixes #5543

[ghost]

This has been released in version 2.0.0 of the provider. Please see the Terraform documentation on provider versioning or reach out if you need any assistance upgrading. As an example:

provider "azurerm" {
    version = "~> 2.0.0"
}
# ... other configuration ...

[ghost]

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