Add dedicated policy for agents started through the vm extension

[narph]

We had some issues with the renaming of Default policies that we can avoid by creating a dedicated policy inside the install process of the vm extension and enrolling the agent to it.

• investigate if this could be done with the current kibana fleet apis
• could be named azure-vm-extension-policy and contain the system integration.
• a check will be done if the policy exists first and then create if not