feat: add a child resource "mysql_flexible_server_aad_administrator"

[kewalaka]

See: https://registry.terraform.io/providers/hashicorp/azurerm/latest/docs/resources/mysql_flexible_server_aad_administrator

I've created an AD group in my tenant that you can use for testing:

group name: azg-mysql-admin objectid: 6c8d236c-3463-479b-9e80-25e3dbda8ca0

[charliekw411]

finished