Option to deactivate the default subset (Service Mesh)

#### Feature Description

I want to disable the default subset in the resolver. This will ensure that if the router cannot determine which subset to send the request to, the request will fail instead of being sent to the default subset.

Currently, the only way to achieve this is by defining a subset with a filter that will never return service instances.

```
Kind = "service-resolver"
Name = "web"
DefaultSubset = "default"
Subsets = {
v1 = {
Filter = "Service.Meta.version == v1"
}
default = {
Filter = "Service.Meta.neverexist == neverexist"
}
}
```