Include customer for server , site and bench

For partners having multiple clients on bench it is useful to have the customer reference on the resources created on frappe.cloud. So having an option for defining a list of customers and having this referenced for sites, benches and servers would enable better management of the environment and automatic billing using the API.

The customer may also be referenced for access restrictions - a certain person may have access to only some customers resources and not others.

Start by tracing how sites, benches, and servers are represented and exposed through the API. Define how a customer list is managed and referenced by those resources, then verify that billing automation and access restrictions can use the customer association.

Written by the indexing model from the issue text.