Add FlowCollector config:

• per-tenant config mode: collect-by-default / reject-by-default / disabled
• defines "admin namespaces" (ie. list/regex of namespaces not concerned by per-tenant config, and still managed directly from the cluster-scope config)

A warning must be displayed if per-tenant is enabled and loki is enabled but not in LokiStack mode (because data is not isolated between tenants in that case)