Label in Web-UI

### Overview of the Issue

Nomad 1.5 added the possibility to add label in the header of the UI.
This helps for example to distinguish a "test"-cluster from a production-cluster.
For consistency reasons it would be good to add it also in consul.

### Reproduction Steps

Go to the UI of consul

### Describe the solution you'd like

Add an option in the configuration of consul to add a label in the header. It would make sense to use the same
options as in Nomad (see https://developer.hashicorp.com/nomad/docs/configuration/ui)

```
label {
text = "production"
background_color = "red"
text_color = "black"
}
```
### Consul Version
v1.15.2

### Screengrabs / Web Inspector logs