Service defaults `MaxFailures` accepts negative numbers

#### Overview of the Issue

`MaxFailures` can be set to a negative integer but it will be converted to an unsigned integer when sent to Envoy.

```hcl
Kind = "service-defaults"
Name = "frontend"
Protocol = "http"
UpstreamConfig {
Defaults {
PassiveHealthCheck {
MaxFailures = -1
}
}
}
```

```console
curl -s localhost:19000/config_dump?format=json | grep -A 2 outlier

"outlier_detection": {
"consecutive_5xx": 4294967295
},
```

### Thoughts

On the one hand, it's doing what I want, which is to essentially disable passive health checking. On the other hand, it seems kinda not good that we're just converting it into an unsigned int.

Maybe we could have `-1` mean "disable outlier detection" and anything other than that throw an error?

### Consul info for both Client and Server

Consul 1.10.3

Start by tracing validation and Envoy conversion for the service-defaults path `UpstreamConfig.Defaults.PassiveHealthCheck.MaxFailures`, then reproduce the negative-value behavior shown by the `/config_dump?format=json` command. Confirm the intended handling of `-1` and other negative values with maintainers; done should include explicit behavior and coverage for the selected validation or conversion outcome.

Written by the indexing model from the issue text.